spice: add 'migrated' flag to spice info

The flag is 'true' when spice migration has completed on the src side.
It is needed for a case where libvirt dies before migration completes
and it misses the event QEVENT_SPICE_MIGRATE_COMPLETED.
When libvirt is restored and queries the migration status, it also needs
to query spice and check if its migration has completed.

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