virtualbox+guest: Update to major version 5.0.0.

The official press release on major changes:

https://www.oracle.com/corporate/pressrelease/oracle-vm-virtualbox-5-070915.html

More details on the changes can be found here:

https://www.virtualbox.org/wiki/Changelog



Built and tested using the Virtualbox NixOS VM test successfully on my
machine but I haven't tested it outside of the NixOS VM test, so please
open an issue if I have fucked up this update.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>