Enable VM contextualization via ISO9660 image

This commit allows to attach an ISO9660 image to a vbox VM
from a well-known file. The purpose is to enable the dynamic
configuration of the VM in first boot so that the volunteer VM
can be kept generic. This process is otherwise widely known as
contextualization of virtual machines.

The actual content of the ISO9660 image is application dependent
and outside the scope of this implementation.

Signed-off-by: Carlos Aguado Sanchez <carlos.aguado@epfl.ch>