Quiesce filesystems with QEMU guest agent during image snapshot

Currently we need to quiesce filesystems (fsfreeze) manually before
snapshotting an image of active instances to create consistent backups.
This proposal is to automate the fsfreeze using QEMU Guest Agent.

Previously-approved: juno
Part of blueprint quiesced-image-snapshots-with-qemu-guest-agent
Change-Id: Ifb77d202e7578b4a379f2894c630bbdda7e9a6a7
Merge "Quiesce filesystems with QEMU guest agent during image snapshot"