Doc libvirt_snapshot_compression option

fixes bug 1111091

libvirt_snapshot_compression -added in grizzly, enables compression
for qcow2 options in libvirt. This patch adds the option to the table.

Change-Id: Ie29ea1f8023c879bf1f6e2fc5bf77dcd1ec9fa61
Merge "Doc libvirt_snapshot_compression option"