Network resource support for libvirt backend

[onixie]

Similar like the PR https://github.com/nix-community/nixops-vbox/pull/2 for virtualbox backend, the changes here are to extend network capabalities for libvirt backend following the idea I proposed at https://github.com/NixOS/nixops/issues/1223.

Though the implementation is quite different from virtualbox backend, I tried to keep the name as well as the layout of nix module options consistent between the two backends. I also add some libvirt specific options such as various modes for macvtap device, virtualport for bridge device such as Open vSwitch, etc.

I have done some simple tests with commands like deploy (and redeploy), start, stop and destroy. I also tried to keep the backward compatibilities though I don't know if my tests could cover all such cases.

Glad to have you review and maybe more tests and trials. Thanks.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/pr-to-support-network-resources-for-virtualbox-backend-in-nixops/6066/2

[grahamc]

Hi there, would you mind rebasing on top of master? There have been some substantial changes to make it work with NixOps master / the upcoming NixOps 2.0.

[onixie]

Hi @grahamc Oh definitely yes. I'll work on it and let you know later.