qemu: Do not poll for spice migration status
    
    QEMU_CAPS_SEAMLESS_MIGRATION capability says QEMU supports
    SPICE_MIGRATE_COMPLETED event. Thus we can just drop all code which
    polls query-spice and replace it with waiting for the event.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>