core: RemoveDiskSnapshot rollback - fetch disk by DiskDao

RemoveDiskSnapshotTaskHandler ->
* endWithFailure: Fetching a disk by imageGroupId should be done using DiskDao.
* endRemoveSnapshotSingleDisk: update TaskGroupSuccess appropriately.

Change-Id: I9b6d811a86c353087479de1fc25276be62df4bec
Bug-Url: https://bugzilla.redhat.com/1134382
Bug-Url: https://bugzilla.redhat.com/1134434
Signed-off-by: Daniel Erez <derez@redhat.com>