nixos/xen: mark qemu as hiPrio

If user already has qemu installed in their system, there might be a collision between normal qemu-system-i386, and qemu-system-i386 with Xen support enabled

In this case, we want Xen supported qemu to win, as otherwise systems won't start with the configuration we provide in NixOS wiki

nixos/modules/virtualisation/xen-dom0.nix

···
 
 let
   inherit (builtins) readFile;
+  inherit (lib.meta) hiPrio;
   inherit (lib.modules) mkRemovedOptionModule mkRenamedOptionModule mkIf;
   inherit (lib.options)
     mkOption
···
     environment = {
       systemPackages = [
         cfg.package
-        cfg.qemu.package
+        (hiPrio cfg.qemu.package)
       ];
       etc =
         # Set up Xen Domain 0 configuration files.