virsh: Allow display of the physical volume size

virsh: Allow display of the physical volume size

Add a new qualifier '--physical' to the 'vol-info' command in order to
dispaly the physical size of the volume. The size can differ from the
allocation value depending on the volume file time. In particular, qcow2
volumes will have a physical value larger than allocation. This also occurs
for sparse files, although for those the capacity is the largest size;
whereas, for qcow2 capacity is the logical size.

Signed-off-by: John Ferlan <jferlan@redhat.com>