Move VirtualBox driver into libvirtd

Move VirtualBox driver into libvirtd

Change the build process & driver initialization so that the
VirtualBox driver is built into libvirtd, instead of libvirt.so
This change avoids the VirtualBox GPLv2-only license causing
compatibility problems with libvirt.so which is under the
GPLv2-or-later license.

NB this change prevents use of the VirtualBox driver on the
Windows platform, until such time as libvirtd can be made
to work there.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>