virtualbox-demo: Add modesetting to drivers

This needs to be included for VirtualBox to detect that it needs to start the video driver. "modesetting" is also set in virtualbox-image.nix but this line seems to take precedence over that one (even though the virtualbox-image.nix has a higher override?) This should fix the problems that I and a few others have been having with the .ova files built for nixos.org.

Fixes #20007.

Changed files
+1 -1
nixos
modules
+1 -1
nixos/modules/installer/virtualbox-demo.nix
···
# Add some more video drivers to give X11 a shot at working in
# VMware and QEMU.
-
services.xserver.videoDrivers = mkOverride 40 [ "virtualbox" "vmware" "cirrus" "vesa" ];
+
services.xserver.videoDrivers = mkOverride 40 [ "virtualbox" "vmware" "cirrus" "vesa" "modesetting" ];
}