Add Vagrant Post-Processor support for Virtualbox OVA artifacts

Adds support for Virtualbox OVA artifacts in Vagrant post-processor, addressing #315. The code extracts the OVA during the artifact copy step in the post-processor, and then proceeds to process the contents of the OVA from that temporary folder (using the same code to process the OVF artifacts).

Code also reworks support functions within `post-processor/vagrant/virtualbox.go` so they can apply in both cases.