Add comment to explain vbm function

As a wrapper around VBoxManage, vbm is a core function of the host-side
scripts, but it lacks a comment explaining what it does and how it is
used.

This patch adds that comment.

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: If3a0a13dd2dc83f031569cf8fc7cd9c604435321
Merge "Add comment to explain vbm function"