virtualbox-image module: enable partition / filesystem growth

nixos/modules/virtualisation/virtualbox-image.nix

···
 
 in {
 
+  imports = [ ./grow-partition.nix ];
+
   options = {
     virtualbox = {
       baseImageSize = mkOption {
···
         '';
     };
 
-    fileSystems."/".device = "/dev/disk/by-label/nixos";
+    fileSystems."/" = {
+      device = "/dev/disk/by-label/nixos";
+      autoResize = true;
+    };
 
     boot.loader.grub.device = "/dev/sda";