Fix error message when TUNNELLED flag is used in non-p2p migration

The current error message is

    error: use virDomainMigrateToURI3 for peer-to-peer migration

which is correct but a bit misleading because the client did not specify
VIR_MIGRATE_PEER2PEER flag. This patch changes the error message to

    error: cannot perform tunnelled migration without using peer2peer
    flag

which is consistent with the error reported by older migration APIs.

Reported by Rich Jones in
https://bugzilla.redhat.com/show_bug.cgi?id=1095924
qemu: Send migrate_cancel when aborting migration

When QEMU reports failed or cancelled migration, we don't need to send
it migrate_cancel QMP command. But in all other error paths, such as if
we detect broken connection to a destination daemon or something else
happens inside libvirt, we need to explicitly send migrate_cancel
command instead of relying on the migration to be implicitly cancelled
when destination QEMU is killed.

Because we were not doing so, one could end up with a paused domain
after failed migration.

https://bugzilla.redhat.com/show_bug.cgi?id=1098833
qemu: Properly abort migration to a file

This is similar to the previous commit in that we need to explicitly
send migrate_cancel when libvirt detects an error other than those
reported by query-migrate. However, the possibility to hit such error is
pretty small.
qemu: process: Refresh backing chain info when reconnecting to qemu

Refresh the disk backing chains when reconnecting to a qemu process
after daemon restart. There are a few internal fields that don't get
refreshed from the XML. Until we are able to do that, let's reload all
the metadata by the backing chain crawler.
qemu: Make qemuDomainPrepareDiskChainElement aware of remote storage

Refactor the function to accept a virStorageSourcePtr instead of just
the path, add a check to run it only on local storage and fix callers
(possibly by using a newly introduced wrapper that wraps a path in the
 virStorageSource struct for legacy code)
storage: Store gluster volume name separately

The gluster volume name was previously stored as part of the source path
string. This is unfortunate when we want to do operations on the path as
the volume is used separately.

Parse and store the volume name separately for gluster storage volumes
and use the newly stored variable appropriately.
storage: Rework debugging of storage file access through storage driver

Print the debug statements of individual file access functions from the
main API functions instead of the individual backend functions.

Also enhance initialization debug messages on a per-backend basis.
conf: Fix domain disk path iterator to work with networked storage

Skip networked storage but continue iteration through backing chain to
iterate through all the local paths in the backing chain.
storage: Add NONE protocol type for network disks

Currently the protocol type with index 0 was NBD which made it hard to
distinguish whether the protocol type was actually assigned. Add a new
protocol type with index 0 to distinguish it explicitly.
storage: Add support for access to files using provided uid/gid

To allow using the storage driver APIs to access files on various
storage sources in a universal fashion possibly on storage such as nfs
with root squash we'll need to store the desired uid/gid in the
metadata.

Add new initialisation API that will store the desired uid/gid and a
wrapper for the current use. Additionally add docs for the two APIs.
storage: Add storage file API to read file headers

Add storage driver based functions to access headers of storage files
for metadata extraction. Along with this patch a local filesystem and
gluster via libgfapi implementation is provided. The gluster
implementation is based on code of the saferead_lim function.
util: new function virTimeLocalOffsetFromUTC

Since there isn't a single libc API to get this value, this patch
supplies one which gets the value by grabbing current time, then
converting that into a struct tm with gmtime_r(), then back to a
time_t using mktime.

The returned value is the difference between UTC and localtime in
seconds. If localtime is ahead of UTC (east) the offset will be a
positive number, and if localtime is behind UTC (west) the offset will
be negative.

This function should be POSIX-compliant, and is threadsafe, but not
async signal safe. If it was ever necessary to know this value in a
child process, we could cache it with a one-time init function when
libvirtd starts, then just supply the cached value, but that
complexity isn't needed for current usage; that would also have the
problem that it might not be accurate after a local daylight savings
boundary.

(If it weren't for DST, we could simply replace this entire function
with "-timezone"; timezone contains the offset of the current timezone
(negated from what we want) but doesn't account for DST. And in spite
of being guaranteed by POSIX, it isn't available on older versions of
mingw.)

Signed-off-by: Eric Blake <eblake@redhat.com>
Revert "qemu: Report the offset from host UTC for RTC_CHANGE event"

This reverts commit e31b5cf393857a6ca78d148c19393e28dfb39de1.

This commit attempted to work around a bug in the offset value
reported by qemu's RTC_CHANGE event in the case that a variable base
date was given on the qemu commandline. The patch mixed up the math
involved in arriving at the corrected offset to report, and in the
process added an unnecessary private attribute to the clock
element. Since that element is private/internal and not used by anyone
else, it makes sense to simplify things by removing it.
qemu: fix RTC_CHANGE event for <clock offset='variable' basis='utc'/>

commit e31b5cf393857 attempted to fix libvirt's
VIR_DOMAIN_EVENT_ID_RTC_CHANGE, which is documentated to always
provide the new offset of the domain's real time clock from UTC. The
problem was that, in the case that qemu is provided with an "-rtc
base=x" where x is an absolute time (rather than "utc" or
"localtime"), the offset sent by qemu's RTC_CHANGE event is *not* the
new offset from UTC, but rather is the sum of all changes to the
domain's RTC since it was started with base=x.

So, despite what was said in commit e31b5cf393857, if we assume that
the original value stored in "adjustment" was the offset from UTC at
the time the domain was started, we can always determine the current
offset from UTC by simply adding the most recent (i.e. current) offset
from qemu to that original adjustment.

This patch accomplishes that by storing the initial adjustment in the
domain's status as "adjustment0". Each time a new RTC_CHANGE event is
received from qemu, we simply add adjustment0 to the value sent by
qemu, store that as the new adjustment, and forward that value on to
any event handler.

This patch (*not* e31b5cf393857, which should be reverted prior to
applying this patch) fixes:

https://bugzilla.redhat.com/show_bug.cgi?id=964177

(for the case where basis='utc'. It does not fix basis='localtime')
qemu: fix <clock offset='variable' basis='localtime'/>

For a clock element as above, libvirt simply converts current system
time with localtime_r(), then starts qemu with a time string that
doesn't contain any timezone information. So, from qemu's point of
view, the -rtc string it gets for:

   <clock offset='variable' basis='utc' adjustment='10800'/>

is identical to the -rtc string it gets for:

   <clock offset='variable' basis='localtime' adjustment='0'/>

(assuming the host is in a timezone that is 10800 seconds ahead of
UTC, as is the case on the machine where this message is being
written).

Since the commandlines are identical, qemu will behave identically
after this point in either case.

There are two problems in the case of basis='localtime' though:

Problem 1) If the guest modifies its RTC, for example to add 20
seconds, the RTC_CHANGE event from qemu will then contain offset:20 in
both cases. But libvirt will have saved the original adjustment into
adjustment0, and will add that value onto the offset in the
event. This means that in the case of basis=;utc', it will properly
emit an event with offset:10820, but in the case of basis='localtime'
the event will contain offset:20, which is *not* the new offset of the
RTC from UTC (as the event it documented to provide).

Problem 2) If the guest is migrated to another host that is in a
different timezone, or if it is migrated or saved/restored after the
DST status has changed from what it was when the guest was originally
started, the newly restarted guest will have a different RTC (since it
will be based on the new localtime, which could have shifted by
several hours).

The solution to both of these problems is simple - rather than
maintaining the original adjustment value along with
"basis='localtime'" in the domain status, when the domain is started
we convert the adjustment offset to one relative to UTC, and set the
status to "basis='utc'". Thus, whatever the RTC offset was from UTC
when it was initially started, that offset will be maintained when
migrating across timezones and DST settings, and the RTC_CHANGE events
will automatically contain the proper offset (which should by
definition always be relative to UTC).

This fixes a problem that was implied but not openly stated in:

  https://bugzilla.redhat.com/show_bug.cgi?id=964177