feat(qcow): add support for specifying contents in a qcow image via specialArgs

[seanrmurphy]

I wanted to be able to inject files into a qcow image using the content parameter in the qcow builder; I just passed it in via specialArgs.

Not really sure if this is the best way to do it, but it worked for me.

Here is an example of how it can be called:

    let
      fs = nixpkgs.lib.fileset;
      sourceFiles = fs.unions [
        ./configuration.nix
        ./flake.nix
        ./README.md
      ];
    in ...
       qcow = nixos-generators.nixosGenerate {
         system = "x86_64-linux";
         modules = [ ./configuration.nix ];
         format = "qcow";
         specialArgs = {
           qcow.contents = [
             {
               source = fs.toSource {
                 root = ./.;
                 fileset = sourceFiles;
               };
               target = "/root/nixos-config";
               user = "root";
               group = "root";
               mode = "755";
             }
           ];
         };

[Mic92]

@Lassulus why we do we now use specialArgs instead of defining cleaner nixos options?

[reinthal]

I was thinking about this also when making pr for #387 Is there a better way of exposing parameters of the upstream imports?

[phaer]

Missed this PR and comment before, but just answered in #387: The "parameters" to upstream imports are often just "module options" - you should be able to just set them in an argument to modules in nixosGenerate.

See #387 (comment) for an example of this.

In this PR though, content is not exposed as an option upstream but just a parameter to the function in make-disk-image.nix. So one would need to lib.mkOption a new one in formats/qcow.nix