Libvirt: Fsfreeze during live-snapshot of qemu/kvm instances

Currently it is required to execute fsfreeze command manually before
taking snapshots of active instances, otherwise the snapshot may be
inconsistent. When qemu-guest-agent is installed in a kvm instance,
we can request the instance to fsfreeze via libvirt's fsFreeze API
which is introduced in libvirt 1.2.5. To utilize this feature,
the image metadata must have 'os_require_quiesce=yes' property.
'hw_qemu_guest_agent=yes' is also needed to enable qemu-guest-agent.

Implements: blueprint quiesced-image-snapshots-with-qemu-guest-agent
Change-Id: I137efe00839ae0cb35e142b0c8f8d75ae61cfb64
Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Merge "Libvirt: Fsfreeze during live-snapshot of qemu/kvm instances"