tap: clear vhost_net backend on cleanup

Frontends calling tap_get_vhost_net get an invalid pointer after the
peer backend has been deleted. Jason Wang <jasowang@redhat.com> reports
this leading to a crash in ack_features when we remove the vhost-net
bakend of a virtio nic.

The fix is simply to clear the backend pointer.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
