nixos/qemu-vm: Rename nixos-disk-image -> nix-store-image

For the image that's actually a store image only.
Helps with visibility. The name "nixos-disk-image" has no special
significance, by grepping nixpkgs.

nixos/modules/virtualisation/qemu-vm.nix

@@ -267,6 +267,7 @@ let
   };
 
   storeImage = import ../../lib/make-disk-image.nix {
+    name = "nix-store-image";
     inherit pkgs config lib;
     additionalPaths = [ regInfo ];
     format = "qcow2";