virtio_net: don't rely on virtqueue_add_buf() returning capacity.

Now we can easily use vq->num_free to determine if there are descriptors
left in the queue, we're about to change virtqueue_add_buf() to return 0
on success.  The virtio_net driver is the only one which actually uses
the return value, so change that.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>