spice: notify spice server on vm start/stop

Spice server needs to know about the vm state in order to prevent
attempts to write to devices when they are stopped, mainly during
the non-live stage of migration.
Instead, spice will take care of restoring this writes, on the migration
target side, after migration completes.

Signed-off-by: Yonit Halperin <yhalperi@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
