virtualbox: force xorg-server-1.18 for now

This is getting a little hacky, but hopefully it won't break anything.

nixos/modules/services/x11/xserver.nix

@@ -459,6 +459,8 @@ in
           knownVideoDrivers;
       in optional (driver != null) ({ inherit name; modules = []; driverName = name; } // driver));
 
+    nixpkgs.config.xorg = optionalAttrs (elem "vboxvideo" cfg.videoDrivers) { abiCompat = "1.18"; };
+
     assertions =
       [ { assertion = config.security.polkit.enable;
           message = "X11 requires Polkit to be enabled (‘security.polkit.enable = true’).";

pkgs/applications/virtualization/virtualbox/guest-additions/default.nix

@@ -4,7 +4,14 @@
 let
   version = virtualbox.version;
   xserverVListFunc = builtins.elemAt (stdenv.lib.splitString "." xorg.xorgserver.version);
-  xserverABI = xserverVListFunc 0 + xserverVListFunc 1;
+
+  # Forced to 1.18 in <nixpkgs/nixos/modules/services/x11/xserver.nix>
+  # as it even fails to build otherwise.  Still, override this even here,
+  # in case someone does just a standalone build
+  # (not via videoDrivers = ["vboxvideo"]).
+  # It's likely to work again in some future update.
+  xserverABI = let abi = xserverVListFunc 0 + xserverVListFunc 1;
+    in if abi == "119" then "118" else abi;
 in
 
 stdenv.mkDerivation {