vms:preseed: disable root user and install salt-minion

The root user is disabled by virtue of setting a random value for the
MD5 hash. The salt config should disable the root account on initial
setup.
vms: enable serial console for virt-install

In order to connect to the serial console to look at installation:

```
$ virsh console debian-7
```
vms:README: add note on connecting to serial console