virtio-net: fix module unloading

Unregister the driver before removing multi-instance hotplug
callbacks. This order avoids the warning issued from
__cpuhp_remove_state_cpuslocked when the number of remaining
instances isn't yet zero.

Fixes: 8017c279

 ("net/virtio-net: Convert to hotplug state machine")
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>