anchor.diff:  
       virtualisation.xen.package = mkOption {
         type = types.package;
  -      default = pkgs.xen;
         defaultText = "pkgs.xen";
         example = literalExample "pkgs.xen-light";
         description = ''
  
positive.code: 
      virtualisation.xen.qemu-package = mkOption {
        type = types.package;
        default = pkgs.xen;
        defaultText = "pkgs.xen";
        example = literalExample "pkgs.qemu_xen-light";
        description = ''
  
negative.code: 
      virtualisation.xen.qemu = mkOption {
        type = types.path;
        defaultText = "\${pkgs.xen}/lib/xen/bin/qemu-system-i386";
        example = literalExample "''${pkgs.qemu_xen-light}/bin/qemu-system-i386";
        description = ''
  