Handle registration of VM with attached storage

[von]

It is possible for various reasons to have an unregistered VM with attached storage. Trying to register such a machine fails:

% pyvbox.py register Base\ WinXP\ VM.xml Error: Unregistered machine 'Base WinXP VM' cannot have storage devices attached (found 2 attachments)

Currently there is no graceful way I know of to make such a VM registratable (the only way I know of is to edit the XML).

Not sure what the solution is, just capturing the problem.