Ensure VDI is created in a sensible location

This commit uses Ruby __FILE__ to ensure that the VDI image for the second
(extra) disc is created in a sensible location, irrespective of where the box is
spun up from. It uses an 'unless' to suppress warnings - as THISLOCATION is not
in use anywhere else, any previous instantiations are most likely erroneous.
Using HERE previously errored/warned too often that it was already instantiated
by the stdlib module.