core: RemoveDiskSnapshots - verify image existence

RemoveDiskSnapshotsCommand -> getImages():
Ensure image existence in DB before adding into images list
(to avoid adding null to the list).

Change-Id: I06fd2062b8c359d2abd7e64e559d1d82197e0d76
Bug-Url: https://bugzilla.redhat.com/1134866
Signed-off-by: Daniel Erez <derez@redhat.com>