This patch seperate the domain driver and the network driver.
libvirt_driver_vbox_impl.la has been linked in the network driver.
So that the version specified codes in vbox_V*.c would only be
compiled once.
The vboxGetNetworkDriver provides a simple interface to get vbox
network driver.
The patch dbb4cbf532 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
