Libvirt: Add support for live snapshots

blueprint libvirt-live-snapshots

Implements support for live snapshots when QEMU 1.3 and Libvirt
1.0.0 are available. This implementation may function with other
hypervisors, at which point _check_use_live_snapshots can be
adjusted accordingly.

At a high level, the process is as follows. First we create a
destination file for the snapshot. This file uses the same backing
file as the origin disk and is of the same virtual size. We do this
since the combination of blockRebase and "shallow" copies are the
only bug-free method available as of QEMU 1.3 (Vish has submitted
patches upstream). This process ultimately produces a CoW file,
representing only the current delta between the root disk and
backing file. The final step converts the CoW delta to a backless
image matching the format of the origin disk.

Flags: DocImpact
Change-Id: Icba53336556295722c3f8b7345e6cd09d8ccfed9

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