vbox: Register per partes

vbox: Register per partes

Since times when vbox moved to the daemon (due to some licensing
issue) the subdrivers that vbox implements were registered, but not
opened since our generic subdrivers took priority. I've tried to fix
this in 65b7d553f39ff9 but it was not correct. Apparently moving
vbox driver registration upfront changes the default connection URI
which makes some users sad. So, this commit breaks vbox into pieces
and register vbox's network and storage drivers first, and vbox driver
then at the end. This way, the vbox driver is registered in the order
it always was, but its subdrivers are registered prior the generic
ones.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>