Added missing translation string
Added missing translation string

Fixes messages like:
```
==> default: translation missing: en.vagrant_libvirt.package_domain
```

electrofelix/vagrant-libvirt,electrofelix/vagrant-libvirt,pradels/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,pradels/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,electrofelix/vagrant-libvirt