linux: enable Xen everywhere it can be

I don't think there's any reason to have a seperate kernel variant
because of this, with all the maintenance burden that imposes.  Debian
and Fedora both enable all these options on their normal kernels.

Alias the Linux Xen attributes, so this change should be seemless for
people who were using the Xen kernels up to now.

All the Xen options are marked as optional anyway, so it should be
fine to try to enable them on non-x86 platforms as well.

Fixes: https://github.com/NixOS/nixpkgs/issues/115182

pkgs/os-specific/linux/kernel/common-config.nix

@@ -12,7 +12,7 @@
 # Configuration
 { lib, stdenv, version
 
-, features ? { grsecurity = false; xen_dom0 = false; }
+, features ? { grsecurity = false; }
 }:
 
 with lib;
@@ -547,13 +547,8 @@ let
       VBOXGUEST = option no;
       DRM_VBOXVIDEO = option no;
 
-    } // optionalAttrs (stdenv.isx86_64 || stdenv.isi686) ({
-      XEN = option yes;
-
-      # XXX: why isn't this in the xen-dom0 conditional section below?
-      XEN_DOM0 = option yes;
-
-    } // optionalAttrs features.xen_dom0 {
+      XEN                         = option yes;
+      XEN_DOM0                    = option yes;
       PCI_XEN                     = option yes;
       HVC_XEN                     = option yes;
       HVC_XEN_FRONTEND            = option yes;
@@ -572,7 +567,7 @@ let
       XEN_SELFBALLOONING          = option yes;
       XEN_STUB                    = option yes;
       XEN_TMEM                    = option yes;
-    });
+    };
 
     media = {
       MEDIA_DIGITAL_TV_SUPPORT = yes;