core: ignore reports of lastStopTime from vdsm

Set VmDynamic#lastStopTime as a field that should not change by VDSM
which will reduce the number of updates in the DB (VmDynamic won't be
updated when nothing relevant was changed)

Change-Id: I5e46b87d8ee1753b77d88ce1987c166e7f2e4509
Bug-Url: https://bugzilla.redhat.com/1042854
Signed-off-by: Arik Hadas <ahadas@redhat.com>