Introduce a bitbake argument and class file that allows VirtualBox Image generation.

Running in a Virtual Machine is a very interesting and appealing value proposition for a number of reasons such as:
 * Preferred option for many for a quick test drive
 * No dedicated HW needed
 * VirtualBox runs on Windows, Mac, Linux
 * Very little upfront investment needed
 * Quick and easy to set-up in hours or less than half a day)
 * Any machine (laptop, desktop) will do
 * Allows to scale development teams to hundreds without the need for each      developer to have access to the target HW