providers/virtualbox: fix out of the box dhcp support

Most users trying DHCP with VirtualBox end up hitting #3083 and become sad.

Here we have two commits which should make them happy again!

Much more detail in the commit messages - happy to tweak and rework as needed - feedback welcome. :cake: 

Looking forward to getting that sucker closed! :dancers: 
Even though these are private methods, it would be really great to have some developer documentation.