make-disk-image: allow vmTools qemu usage to be overriden by disko configuration

This is needed to prevent two layers of emulation when offering a build for a host running binfmt such as an x86 host trying to compile for aarch64 via binfmt

lib/make-disk-image.nix

@@ -9,6 +9,7 @@
 let
   vmTools = pkgs.vmTools.override {
     rootModules = [ "9p" "9pnet_virtio" "virtio_pci" "virtio_blk" ] ++ nixosConfig.config.disko.extraRootModules;
+    customQemu = nixosConfig.config.disko.imageBuilderQemu;
     kernel = pkgs.aggregateModules
       (with nixosConfig.config.disko.imageBuilderKernelPackages; [ kernel ]
         ++ lib.optional (lib.elem "zfs" nixosConfig.config.disko.extraRootModules) zfs);

module.nix

@@ -10,8 +10,16 @@ let
 in
 {
   options.disko = {
-    imageBuilderKernel = lib.mkOption {
-      type = lib.types.package;
+    imageBuilderQemu = lib.mkOption {
+      type = lib.types.nullOr lib.types.str;
+      description = ''
+        the qemu emulator string used when building disk images via make-disk-image.nix.
+        Useful when using binfmt on your build host, and wanting to build disk
+        images for a foreign architecture
+      '';
+      default = null;
+      example = lib.literalExpression "''${pkgs.qemu_kvm}/bin/qemu-system-aarch64";
+    };
     imageBuilderPkgs = lib.mkOption {
       type = lib.types.attrs;
       description = ''