Custom disk directory in infrastructure.libvirt.domains

Currently this is assumed to be under "topdir":

```
{%- set root_disk = topdir ~ '/disks/' ~ machine ~ '_root.qcow2' %}
```

We now have machines where the VM disk images should be located on a separate host disk, hence this should be configurable through the pillar (whilst defaulting to the existing standard).

For the time being, a bind mount works nicely though.