Allow running libvirtd deployments on remote hosts

[mbrgm]

It would be helpful to have the ability to run a libvirtd deployment on a remote host.

[jhillyerd]

This may already be possible, see:

• https://github.com/NixOS/nixops/pull/824
• https://github.com/nix-community/nixops-libvirtd/blob/master/nix/libvirtd.nix#L34

I can try testing and submit some docs if it works.