use the nixpkgs xen module

nixos/roles/default.nix

@@ -1,9 +1,9 @@
-{pkgs, config, lib, ...} : {
+{pkgs, config, lib, modulesPath, ...} : {
 
 imports = [
   ./admins.nix
   ./nix_keys.nix
-  ./xen_guest.nix
+  (modulesPath + "/virtualisation/xen-domU.nix")
   ];
 nix = {
   extraOptions = ''

nixos/roles/xen_guest.nix

@@ -1,16 +0,0 @@
-{...}: {
-  imports = [ ];
-  boot = {
-    loader.grub = {
-      device = "nodev";
-      enable = true;
-    };
-    initrd = {
-      availableKernelModules = [ "ata_piix" "sr_mod" "xen_blkfront" ];
-      kernelModules = [ ];
-    };
-    extraModulePackages = [ ];
-    tmp.useTmpfs = true;
-  };
-}
-