libvirt: add hyperv driver

[jakobrs]

Motivation for this change

To add Hyper-V support to libvirt

Things done

It builds, and virsh --connect hyperv://host/ no longer complains, but I haven't tested connecting to an actual Hyper-V server yet.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
  • Increase in closure size: 1.7M
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[ryantm]

It seems wrong that HyperV is enabled by default when the rest of the drivers are not. Personally, I'd rather have the default be they are all enabled and then if someone needs a smaller size, they would disable them. However, I don't know much about this package.

cc @volth @fpletz

[SuperSandro2000]

Please disable this by default because this will be used by a very tiny amount of people.

[ryantm]

I was not able to use this to connect to a Hyper-V 2016 server. This may or may not be related to how this bug report says the hyper-v module is unmaintained: https://bugzilla.redhat.com/show_bug.cgi?id=1253265

[SuperSandro2000]

Please disable this by default because this will be used by a very tiny amount of people.

[jakobrs]

It might be worth verifying that libvirt's hyperv driver actually works before merging. I tried to do this (with a Windows 10 guest running Hyper-V), but I was unable to successfully enable Hyper-V on the guest. (It got stuck in automatic repair)