virtualbox: Comply with the Personal Use and Evaluation License

The direct download was unfortunately not compliant with the VirtualBox
Extension Pack's Personal Use and Evaluation License (PUEL) which
stipulates that any end-user should fetch the binaries from the official
Downloads page.

See https://github.com/NixOS/nixpkgs/issues/672 for the original issue and
http://www.virtualbox.org/wiki/VirtualBox_PUEL for more info.