The patch dbb4cbf5 by Michal has splited the vbox driver into
three parties. This modification brings a more suitable interface
to the previous patch.

The new function vboxGetDriver is introduced to get the
corresponding vbox domain driver directly thought the vbox version.

Functions like vboxGetNetworkDriver and vboxGetStorageDriver
will be introduced after rewriting it's drivers.

This patch, by the way, fixed the align problem for vbox in
Makefile.am