block: raw-posix image file reopen

This is derived from the Supriya Kannery's reopen patches.

This contains the raw-posix driver changes for the bdrv_reopen_*
functions.  All changes are staged into a temporary scratch buffer
during the prepare() stage, and copied over to the live structure
during commit().  Upon abort(), all changes are abandoned, and the
live structures are unmodified.

The _prepare() will create an extra fd - either by means of a dup,
if possible, or opening a new fd if not (for instance, access
control changes).  Upon _commit(), the original fd is closed and
the new fd is used.  Upon _abort(), the duplicate/new fd is closed.
block: raw image file reopen

These are the stubs for the file reopen drivers for the raw format.

There is currently nothing that needs to be done by the raw driver
in reopen.
block: qed image file reopen

These are the stubs for the file reopen drivers for the qed format.

There is currently nothing that needs to be done by the qed driver
in reopen.
block: qcow2 image file reopen

These are the stubs for the file reopen drivers for the qcow2 format.

There is currently nothing that needs to be done by the qcow2 driver
in reopen.
block: qcow image file reopen

These are the stubs for the file reopen drivers for the qcow format.

There is currently nothing that needs to be done by the qcow driver
in reopen.
block: vmdk image file reopen

This patch supports reopen for VMDK image files.  VMDK extents are added
to the existing reopen queue, so that the transactional model of reopen
is maintained with multiple image files.
block: raw-win32 driver reopen support

This is the support for reopen, for win32.  Note that there is a key
difference in the win32 reopen, namely that it is not guaranteed safe
like all the other drivers.  Attempts are made to reopen the file, or
open the file in a new handle prior to closing the old handle.  However,
this is not always feasible, and there are times when we must close the
existing file and then open the new file, breaking the transactional nature
of the reopen.
block: vdi image file reopen

There is currently nothing that needs to be done for VDI reopen.
block: vpc image file reopen

There is currently nothing that needs to be done for VPC image
file reopen.
block: convert bdrv_commit() to use bdrv_reopen()

Currently, bdrv_commit() reopens images r/w itself, via risky
_delete() and _open() calls. Use the new safe method for drive reopen.
block: remove keep_read_only flag from BlockDriverState struct

The keep_read_only flag is no longer used, in favor of the bdrv
flag BDRV_O_ALLOW_RDWR.
block: add support functions for live commit, to find and delete images.

Add bdrv_find_overlay(), and bdrv_drop_intermediate().

bdrv_find_overlay():  given 'bs' and the active (topmost) BDS of an image chain,
                    find the image that is the immediate top of 'bs'

bdrv_drop_intermediate():
                    Given 3 BDS (active, top, base), delete images above
                    base up to and including top, and set base to be the
                    parent of top's child node.

                    E.g., this converts:

                    bottom <- base <- intermediate <- top <- active

                    to

                    bottom <- base <- active
block: add live block commit functionality

This adds the live commit coroutine.  This iteration focuses on the
commit only below the active layer, and not the active layer itself.

The behaviour is similar to block streaming; the sectors are walked
through, and anything that exists above 'base' is committed back down
into base.  At the end, intermediate images are deleted, and the
chain stitched together.  Images are restored to their original open
flags upon completion.
blockdev: rename block_stream_cb to a generic block_job_cb
qerror: new error for live block commit, QERR_TOP_NOT_FOUND
block: helper function, to find the base image of a chain

This is a simple helper function, that will return the base image
of a given image chain.
QAPI: add command for live block commit, 'block-commit'

The command for live block commit is added, which has the following
arguments:

device: the block device to perform the commit on (mandatory)
base:   the base image to commit into; optional (if not specified,
        it is the underlying original image)
top:    the top image of the commit - all data from inside top down
        to base will be committed into base. optional (if not specified,
        it is the active image) - see note below
speed:  maximum speed, in bytes/sec
on_error: action to take on error (optional - default is report)

note: eventually this will support merging down the active layer,
      but that code is not yet complete.  If the active layer is passed
      in currently as top, or top is left to the default, then the error
      QERR_TOP_NOT_FOUND will be returned.

The is done as a block job, so upon completion a BLOCK_JOB_COMPLETED will
be emitted.
qemu-iotests: add initial tests for live block commit

Derived from the streaming test cases (030), this adds the
following tests:

1. For the following image chain, commit [mid] into [backing],
   and use qemu-io to verify [backing] has its original data, as
   well as the data from [mid]

           [backing] <-- [mid] <-- [test]

2. Verifies that 'block-commit' with the 'speed' parameter sets the
   speed parameter, as reported by 'query-block-jobs'

3. Verifies that a bogus 'device' parameter to 'block-commit'
   results in error
block: after creating a live snapshot, make old image read-only

Currently, after a live snapshot of a drive, the image that has
been 'demoted' to be below the new active layer remains r/w.
This patch reopens it read-only.

Note that we do not check for error on the reopen(), because we
will not abort the snapshots if the reopen fails.

This patch depends on the bdrv_reopen() series.
temp to make compile
(cherry picked from commit 1c7d05a7b1c1ea6aacedb69c80e9489b231dc90e)