build: avoid obsolete AC_HELP_STRING

Autoconf states that AC_HELP_STRING is obsolete, and that new
programs should use AS_HELP_STRING.  We also had instances of
not properly quoting the macro usage, and not relying on autoconf's
word-wrapping abilities to avoid long lines.  I validated that this
commit has no impact to the generated configure file.

* configure.ac (AC_ARG_WITH, AC_ARG_ENABLE): Autoconf recommends
the use of AS_HELP_STRING.  Also, use proper quoting and wrap long
lines.
* m4/virt-apparmor.m4 (LIBVIRT_CHECK_APPARMOR): Likewise.
* m4/virt-selinux.m4 (LIBVIRT_CHECK_SELINUX): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>
build: fix regression in requiring yajl for new enough qemu

Jonathan Lebon reported an issue to me off-list about his build
failing to use qemu because he failed to install yajl-devel.  But
I recalled specifically tweaking configure.ac to die in that
situation (commits 350583c, ba9c38b).  After a bit more
head-scratching, we found the cause of the regression: commit
654c709 rearranged things so that the qemu version check now
occurs before AC_ARG_WITH has had a chance to set either
$with_qemu or $with_yajl.

Coincidentally, this fix aligns with a documentation patch that
was just posted to the autoconf mailing list :)
http://thread.gmane.org/gmane.comp.sysutils.autoconf.patches/8324

* m4/virt-lib.m4 (LIBVIRT_CHECK_LIB, LIBVIRT_CHECK_LIB_ALT)
(LIBVIRT_CHECK_PKG): Populate defaults earlier.
* configure.ac (AC_ARG_WITH): Likewise for drivers.

Signed-off-by: Eric Blake <eblake@redhat.com>
virFileFsType: get filesystem type of a given path

This can be used by storage pools to figure out which actions are available
on various paths (for example subvolumes when running on btrfs.)

Signed-off-by: Oskari Saarenmaa <os@ohmu.fi>
storage: btrfs subvolumes for directory pools

This commit adds support for btrfs subvolumes as directory volumes.  The
directory storage pool can be used to manage btrfs subvolumes on an existing
btrfs filesystem.  The driver can create new blank subvolumes and snapshots
of existing subvolumes as well as delete existing subvolumes.

The subvolumes created are automatically made visible on the host side
and can be attached to domains using the <filesystem> tags as defined in
'format domain' documentation.

Subvolumes do not implement quotas at the moment because the current
(btrfs-progs-0.20.rc1.20130501git7854c8b-4.fc20.x86_64) support for quota
management in btrfs-progs is lacking the necessary features, for example
it's not possible to see the quota assigned to a certain subvolume and
usage information is only updated on syncfs(2).  Quota support will be
implemented once the tools gain the necessary features.

Signed-off-by: Oskari Saarenmaa <os@ohmu.fi>