lguest: clean up length-used value in example launcher

The "len" field in the used ring for virtio indicates the number of
bytes *written* to the buffer.  This means the guest doesn't have to
zero the buffers in advance as it always knows the used length.

Erroneously, the console and network example code puts the length
*read* into that field.  The guest ignores it, but it's wrong.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Documentation/lguest/lguest.c

@@ -830,15 +830,14 @@ static bool handle_console_input(struct device *dev)
 static void handle_console_output(struct virtqueue *vq, bool timeout)
 {
 	unsigned int head, out, in;
-	int len;
 	struct iovec iov[vq->vring.num];
 
 	/* Keep getting output buffers from the Guest until we run out. */
 	while ((head = get_vq_desc(vq, iov, &out, &in)) != vq->vring.num) {
 		if (in)
 			errx(1, "Input buffers in output queue?");
-		len = writev(STDOUT_FILENO, iov, out);
-		add_used_and_trigger(vq, head, len);
+		writev(STDOUT_FILENO, iov, out);
+		add_used_and_trigger(vq, head, 0);
 	}
 }
 
@@ -870,7 +869,6 @@ static void block_vq(struct virtqueue *vq)
 static void handle_net_output(struct virtqueue *vq, bool timeout)
 {
 	unsigned int head, out, in, num = 0;
-	int len;
 	struct iovec iov[vq->vring.num];
 	static int last_timeout_num;
 
@@ -878,10 +876,9 @@ static void handle_net_output(struct virtqueue *vq, bool timeout)
 	while ((head = get_vq_desc(vq, iov, &out, &in)) != vq->vring.num) {
 		if (in)
 			errx(1, "Input buffers in output queue?");
-		len = writev(vq->dev->fd, iov, out);
-		if (len < 0)
+		if (writev(vq->dev->fd, iov, out) < 0)
 			err(1, "Writing network packet to tun");
-		add_used_and_trigger(vq, head, len);
+		add_used_and_trigger(vq, head, 0);
 		num++;
 	}