https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/virtualisation/virtualbox-guest.nix

2020
https://www.reddit.com/r/NixOS/comments/ga86ol/running_virtualbox_on_2003_as_user/
    # /etc/nixos/configuration.nix
    virtualisation.virtualbox.host.enable = true;
    virtualisation.virtualbox.host.enableExtensionPack = true;

    users.extraGroups.vboxusers.members = ["sketchet"];

    # This is required for the ExtensionPack
    nixpkgs.config.allowUnfree = true;

misc
virtualisation.virtualbox.guest.enable=true;
    https://www.reddit.com/r/NixOS/comments/8brb47/correct_way_to_install_virtualbox_guest_additions/