s390x: no deprecation warning while testing

'make check' tries to start all available machines; the deprecation
message for the s390-virtio machine is both useless and annoying
there. Silence it while testing.

Reported-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
