#21 configurable vbox_folder for using it on OSX

[Martin Weißhaupt]

To make the VBoxTool work with OS X (and possible other systems) it would be very good to make the vbox_folder configurable.

For example the logfiles are saved to $HOME/.VirtualBox now but the correct folder on OS X would be $HOME/Library/VirtualBox.
For the backward compatibility it would be the best to stick to the old behavior if no vbox_folder is specified in the vboxtool.conf