libvirt with modified QEMU and OVMF

["Lars Böckel" <listdump@larf.space>]

Hello Guix,

i wanted to recommend some changes to libvirt which would improve the 
usability of libvirt.

1) OVMF(UEFI) capability with QEMU. In NixOS you can achieve this with

   virtualisation.libvirtd.enable = true;
   virtualisation.libvirtd.qemuOvmf = true;
    virtualisation.libvirtd.qemuVerbatimConfig = ''
      nvram = [ "${pkgs.OVMF}/FV/OVMF.fd:${pkgs.OVMF}/FV/OVMF_VARS.fd" ]
      user = "1000"
    '';

the "user" part is not necessary but is required for system VMs in 
combination with qemu's pulseaudio driver. qemuVerbatimConfig is the 
config that is at /etc/libvirt/qemu.conf in traditional GNU/Linux 
distributions.

2) Using a modified QEMU with libvirt. A modified QEMU could help 
sometimes. A way to specify which QEMU package to use would be helpful 
to resolve issues with newer qemu versions or patches which are not 
upstream. While in NixOS you could add patches like this

     nixpkgs.config.packageOverrides = pkgs: rec {
     qemu = pkgs.qemu.overrideAttrs (attrs:{
       patches = (lib.filter (x: ! (lib.hasSuffix 
"fix-hda-recording.patch" (builtins.toString x)))  attrs.patches) ++ [ 
/path/to/qemu.patch    ];
        }
     );
   };

to the QEMU package, i don't think this is possible in GuixSD.

Thank you for your work. Please let me know what you think.