core: remove snapshot's memory on remove vm from export domain

When a VM in export domain is being removed, the volumes of the memory
of its snapshots should be removed from the export domain as well.

Change-Id: I0bf0f29aade5cb7c93949a27067d7c4270e86032
Bug-Url: https://bugzilla.redhat.com/960931
Signed-off-by: Arik Hadas <ahadas@redhat.com>