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.