azure-image: add an internal memSize option to workaround potential OOM

nixos/modules/virtualisation/azure-image.nix

@@ -67,6 +67,16 @@ in
         For v2, secure boot needs to be turned off during creation.
       '';
     };
+
+    memSize = mkOption {
+      type = types.nullOr types.int;
+      default = null;
+      internal = true;
+      description = ''
+        Guest memory allocation size during image generation, in MiB.
+        Only change this when you face Out-Of-Memory issue in QEMU when generating image.
+      '';
+    };
   };
 
   config = {
@@ -89,7 +99,7 @@ in
       bootSize = "${toString cfg.bootSize}M";
       partitionTableType = if (cfg.vmGeneration == "v2") then "efi" else "legacy";
 
-      inherit (cfg) contents label;
+      inherit (cfg) contents label memSize;
       inherit (config.virtualisation) diskSize;
       inherit config lib pkgs;
     };