libvirtd: If baseImage default is null, the option type must allow it.

@rbvermaa This fixes the following nixops runtime error for me (introduced by 9d41f59e827f25ca9d693753dcfd281ce3ab6615, I guess):

```
The option value `deployment.libvirtd.baseImage' in `/nix/store/zhf89zp8w0hf85yvr1k1zs4ix3f0r0df-nixops-1.3pre0_abcdef/share/nix/nixops/libvirtd.nix' is not a path.
```