virtio: console makes incorrect assumption about virtio API

The get_buf() API sets the second arg to the number of bytes *written*
by the other side; in this case it should be zero as these are output buffers.

lguest gets this right (obviously kvm's console doesn't), resulting in
continual buildup of console writes.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Amit Shah <amit.shah@redhat.com>

drivers/char/virtio_console.c

@@ -416,20 +416,16 @@ static ssize_t send_buf(struct port *port, void *in_buf, size_t in_count)
 	out_vq->vq_ops->kick(out_vq);
 
 	if (ret < 0) {
-		len = 0;
+		in_count = 0;
 		goto fail;
 	}
 
-	/*
-	 * Wait till the host acknowledges it pushed out the data we
-	 * sent. Also ensure we return to userspace the number of
-	 * bytes that were successfully consumed by the host.
-	 */
+	/* Wait till the host acknowledges it pushed out the data we sent. */
 	while (!out_vq->vq_ops->get_buf(out_vq, &len))
 		cpu_relax();
 fail:
 	/* We're expected to return the amount of data we wrote */
-	return len;
+	return in_count;
 }
 
 /*