vhost: disable on tap link down

qemu makes it possible to disable link at tap which is not communicated
to the guest but causes all packets to be dropped.

When vhost-net is enabled, vhost needs to be aware of both the virtio
link_down and the peer link_down. we switch to userspace emulation when
either is down.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reported-by: Npradeep <psuriset@linux.vnet.ibm.com>
Acked-by: NAlex Williamson <alex.williamson@redhat.com>
Acked-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

hw/virtio-net.c

@@ -115,7 +115,8 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t status)
     if (!tap_get_vhost_net(n->nic->nc.peer)) {
         return;
     }
-    if (!!n->vhost_started == virtio_net_started(n, status)) {
+    if (!!n->vhost_started == virtio_net_started(n, status) &&
+                              !n->nic->nc.peer->link_down) {
         return;
     }
     if (!n->vhost_started) {