core: DiskImage - added vmSnapshotDescription

Added vmSnapshotDescription member/setter/getter to
DiskImage to accompany the existing vmSnapshotId
(needed for displaying snapshot's descritopn in client).

Change-Id: Ic658ab2603a8033af7ddd6bd57b34b880f34b393
Signed-off-by: Daniel Erez <derez@redhat.com>