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)

In addition "id" and "migratingToVds" fields in VmDynamic will also be
marked with the UnchangeableByVdsm annotation as they should not be
changed by VDSM as well

Change-Id: I5e46b87d8ee1753b77d88ce1987c166e7f2e4509
Signed-off-by: Arik Hadas <ahadas@redhat.com>