virtio-net: fix removal of failover device

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
77b06bba62034a87cc61a9c8de1309ae3e527d97

somehow papers over it.

CC: Jason Wang <jasowang@redhat.com>
CC: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>