engine: Removes unused image groups after snapshot removal

Currently snapshot removal process keeps leftovers in the storage
domain. For example let's take the following scenario:
1 VM with 1 Disk
Now we creating the first snapshot
Add another disk to VM
Now let's go back preview and commit the first snapshot
At this point second disk is non reachable and not used any more, but
its left on the storage domain.
This patch initializes removal of these leftovers.

Change-Id: I07928bc8ff4225055586b96e47d3787d741a95b4
Bug-Url: https://bugzilla.redhat.com/987994
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>