virtio: Complete feature negotation before updating status

lguest (in rusty's use-tun-ringfd patch) assumes that the
guest has updated its feature bits before setting its status
to VIRTIO_CONFIG_S_DRIVER_OK.

That's pretty reasonable, so let's make it so.
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/virtio/virtio.c

@@ -124,9 +124,9 @@ static int virtio_dev_probe(struct device *_d)
 	if (err)
 		add_status(dev, VIRTIO_CONFIG_S_FAILED);
 	else {
-		add_status(dev, VIRTIO_CONFIG_S_DRIVER_OK);
 		/* They should never have set feature bits beyond 32 */
 		dev->config->set_features(dev, dev->features[0]);
+		add_status(dev, VIRTIO_CONFIG_S_DRIVER_OK);
 	}
 	return err;
 }