If the image gets registered as public allow anyone to read the
metadata and the md5sum file.

Rename DiskDevice class to Image and move it to a seperate module.
Also, don't let the user access the os_type/* modules directly. In
Image class create the `os' member variable that will host an
appropriate instance of one of the OSBase classes.

Create a new dialog_menu module to host all the "expert mode" code.
Check the code with a new version of pep8