virtio-net: fix removal of failover device

If you have a networking device and its virtio failover device, and
you remove them in this order:
- virtio device
- the real device

You get qemu crash.
See bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1820120

Bug exist on qemu 4.2 and 5.0.
But in 5.0 don't shows because commit
77b06bb

somehow papers over it.

CC: Jason Wang <[email protected]>
CC: Michael S. Tsirkin <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Acked-by: Laurent Vivier <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>
Signed-off-by: Jason Wang <[email protected]>

Author: Juan Quintela

hw/net/virtio-net.c

@@ -3416,6 +3416,7 @@ static void virtio_net_device_unrealize(DeviceState *dev)
     g_free(n->vlans);
 
     if (n->failover) {
+        device_listener_unregister(&n->primary_listener);
         g_free(n->primary_device_id);
         g_free(n->standby_id);
         qobject_unref(n->primary_device_dict);