diskSize expects string

[jaroeichler]

The example to adjust the diskSize in the readme doesn't work for me. nix build expects a string like this:

test = nixos-generators.nixosGenerate (
  config-test
  // {
    format = "vm-nogui";
    specialArgs = { diskSize = "20000"; };
  }
);

In particular, it fails with something like "20 * 1024".