core: handling memory snapshots on export/import vm

Export the memory volumes to the export domain when exporting VM that
has snapshots that contain memory, and importing the memory volumes when
importing VM that has snapshots that contain memory.

Change-Id: Ifae3d5dd6bdeca04d5b5c9eee2dbe93e3eb03cea
Bug-Url: https://bugzilla.redhat.com/960931
Signed-off-by: Arik Hadas <ahadas@redhat.com>