core: update DiskImageDynamic on RemoveSnapshot

* Fetching image's size on endSuccessfully of
  RemoveSnapshotSingleDiskCommand.
* Updating DiskImageDynamic with the retrieved value.

Change-Id: Ie124fb42c34860ee349e91172e0248a2d824b127
Bug-Url: https://bugzilla.redhat.com/923864
Signed-off-by: Daniel Erez <derez@redhat.com>