virtualbox: Allow building with extension pack.

The extension pack is needed in order to have USB 2.0, RDP and PXE boot support
and is _not_ part of the free version, so please read their conditions before
using it:

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

Further information and details about the extension pack is available here:

https://www.virtualbox.org/manual/ch01.html#intro-installing



I'm wimping out here and use requireFile to ask the user to fetch the extension
pack manually, simply because I'm not a lawyer. I've seen other distributions
that fetch the extension packs directly from upstream, but I'm not sure if that
is according to Oracle's PUEL.

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