Fix bug in MTU declaration.

According to Andrea, it was printing a network line that looked like this:

```
--mtu=9000 network --bootproto=static --ip=131.154.192.11
--netmask=255.255.252.0 --gateway=131.154.192.1
--nameserver=131.154.128.2 --device=eth0
--hostname=wn-test.cr.cnaf.infn.it
```

We now print the mtu field at the end of the line!