Libvirt: Implement snapshots for LVM-backed roots

Uses the built-in LVM snapshotting mechanism to create a new
snapshot, use qemu-img to extract the full root into an image file,
then delete the snapshot volume. Although we typically zero deleted
LVM volumes manually, this is not necessary for snapshots, as LVM
handles it for us (per the documentation).

Change-Id: I82f21488671ffa3be3d0feba73ff631fa2a51ec0
Fixes: Bug 1120809
Flags: DocImpact
Merge "Libvirt: Implement snapshots for LVM-backed roots"