disk-image: Fix "unexpected argument 'customQemu'" eval error

Previously, an error like

    error: function 'anonymous lambda' called with unexpected argument 'customQemu'

    at /nix/store/lbqj1cndic4121whnx8xm0jgb1c8x4xx-source/pkgs/build-support/vm/default.nix:1:1:

was printed when trying to evaluate `config.system.build.vmWithDisko` or
`config.system.build.diskoImagesScript` with nixpkgs 24.05 (and below).

This argument was added in
65c851cd75,
so technically the minimum version is 24.11.20240708.65c851c. However,
`versionAtLeast` only compares versions alphabetically, so the comparison's
result will be incorrect for other commits made on the same day.

Instead, we compare against 24.11.20240709, which is the next day.
This means this function returns false for some commits that DO support the
customQemu argument, but if it returns true, we can be 100% certain that this
is correct, and we can pass the customQemu argument to vmTools without an
evaluation error.

Fixes #850

module.nix

@@ -194,6 +194,17 @@ in
   };
 
   config = {
+    assertions = [
+      {
+        assertion = config.disko.imageBuilder.qemu != null -> diskoLib.vmToolsSupportsCustomQemu pkgs;
+        message = ''
+          You have set config.disko.imageBuild.qemu, but vmTools in your nixpkgs version "${lib.version}"
+          does not support overriding the qemu package with the customQemu option yet.
+          Please upgrade nixpkgs so that `lib.version` is at least "24.11.20240709".
+        '';
+      }
+    ];
+
     _module.args.diskoLib = import ./lib {
       inherit lib;
       rootMountPoint = config.disko.rootMountPoint;
@@ -226,8 +237,11 @@ in
 
     # we need to specify the keys here, so we don't get an infinite recursion error
     # Remember to add config keys here if they are added to types
-    fileSystems = lib.mkIf cfg.enableConfig cfg.devices._config.fileSystems or { };
-    boot = lib.mkIf cfg.enableConfig cfg.devices._config.boot or { };
-    swapDevices = lib.mkIf cfg.enableConfig cfg.devices._config.swapDevices or [ ];
+    fileSystems = lib.mkIf
+      cfg.enableConfig cfg.devices._config.fileSystems or { };
+    boot = lib.mkIf
+      cfg.enableConfig cfg.devices._config.boot or { };
+    swapDevices = lib.mkIf
+      cfg.enableConfig cfg.devices._config.swapDevices or [ ];
   };
 }