configure-network can now add port-forwarding rules to adapters.

Example: to configure an adapter that port forwards the host's port 8080 to the
guest's port 80:

```clojure
(configure-network machine {:attachment-type :nat
                            :nat-rules [{:name "http", :protocol :tcp,
                                         :host-ip "", :host-port 8080,
                                         :guest-ip "", :guest-port 80}]})
```