virtio: Fix return value for dummy function vhost_net_virtqueue_pending

cgcc complains that -ENOSYS is not a good value for 'bool'.

A dummy virtio will never have pending queue entries, so let us return
false.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/net/vhost_net.c

@@ -321,7 +321,7 @@ void vhost_net_ack_features(struct vhost_net *net, unsigned features)
 
 bool vhost_net_virtqueue_pending(VHostNetState *net, int idx)
 {
-    return -ENOSYS;
+    return false;
 }
 
 void vhost_net_virtqueue_mask(VHostNetState *net, VirtIODevice *dev,