Add support for compressing qcow2 snapshots

Fixes bug 1109923

Adds a new configuration option:
    libvirt_snapshot_compression=False (Default)

When set to True and the snapshot output format is qcow2, we pass
the -c option into qemu-img, which enables compression. This patch
also refactors the extract_image function to make construction of
the whole qemu-img command more dynamic.

Change-Id: I7a7ebcf41a91a8a9cb14be79e9bb79c22acb136e
Flags: DocImpact
Merge "Add support for compressing qcow2 snapshots"