virtio_ccw: support non-legacy balloon devices

As of last patch, virtio_device_is_legacy_only is always false,
drop the test from virtio ccw.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (slight reword)