make-single-disk-zfs-image: make memSize configurable

Signed-off-by: Sirio Balmelli <sirio@b-ad.ch>

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

@@ -21,6 +21,9 @@
 , # size of the FAT partition, in megabytes.
   bootSize ? 1024
 
+  , # memory allocated for virtualized build instance
+  memSize ? 1024
+
 , # The size of the root partition, in megabytes.
   rootSize ? 2048
 
@@ -230,7 +233,7 @@ let
   ).runInLinuxVM (
     pkgs.runCommand name
       {
-        memSize = 1024;
+        inherit memSize;
         QEMU_OPTS = "-drive file=$rootDiskImage,if=virtio,cache=unsafe,werror=report";
         preVM = ''
           PATH=$PATH:${pkgs.qemu_kvm}/bin