nixos/virtualbox-image: Fix path to virtualbox

VirtualBox user space binaries now no longer reside in linuxPackages, so
let's use the package for the real user space binaries instead.

Tested using the following command:

nix-build nixos/release.nix -A ova.x86_64-linux

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