virtio-net: drop assert on vm stop

On vm stop, vm_running state set to stopped
before device is notified, so callbacks can get envoked with
vm_running = false; and this is not an error.

Cc: qemu-stable@nongnu.org
Acked-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 131c5221)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hw/net/virtio-net.c

@@ -1125,8 +1125,6 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
         return num_packets;
     }
 
-    assert(vdev->vm_running);
-
     if (q->async_tx.elem.out_num) {
         virtio_queue_set_notification(q->tx_vq, 0);
         return num_packets;