nixos/lib/make-multi-disk-zfs-image: expose memSize parameter

closes #178095

nixos/lib/make-multi-disk-zfs-image.nix

@@ -73,6 +73,9 @@
 , # Shell code executed after the VM has finished.
   postVM ? ""
 
+, # Guest memory size
+  memSize ? 1024
+
 , name ? "nixos-disk-image"
 
 , # Disk image format, one of qcow2, qcow2-compressed, vdi, vpc, raw.
@@ -242,6 +245,7 @@ let
       {
         QEMU_OPTS = "-drive file=$bootDiskImage,if=virtio,cache=unsafe,werror=report"
          + " -drive file=$rootDiskImage,if=virtio,cache=unsafe,werror=report";
+         inherit memSize;
         preVM = ''
           PATH=$PATH:${pkgs.qemu_kvm}/bin
           mkdir $out