vhost: add unlikely annotations to error path

patch 'break out of polling loop on error' caused
a minor performance regression on my machine: recover
that performance by adding a bunch of unlikely annotations
in the error handling.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/net.c

@@ -137,7 +137,7 @@ static void handle_tx(struct vhost_net *net)
 					 &out, &in,
 					 NULL, NULL);
 		/* On error, stop handling until the next kick. */
-		if (head < 0)
+		if (unlikely(head < 0))
 			break;
 		/* Nothing new?  Wait for eventfd to tell us they refilled. */
 		if (head == vq->num) {
@@ -234,7 +234,7 @@ static void handle_rx(struct vhost_net *net)
 					 &out, &in,
 					 vq_log, &log);
 		/* On error, stop handling until the next kick. */
-		if (head < 0)
+		if (unlikely(head < 0))
 			break;
 		/* OK, now we need to know about added descriptors. */
 		if (head == vq->num) {