Cleanup the dialog_* modules

 * Cleanup Wizard and WizardPage classes in dialog_wizard
 * Fix various pylint warnings
windows: Add traverse function in Registry

This can be used to fetch a registry node by defining a registry path
windows: Change how powershell module is imported

Import the module itself and not every single constant that is
defined there.
windows: Enumarate members of the Admin. group

In Windows, when enumerating active users, also enumerate users that
are members of the Administrators group.
Fix a closed libguestfs handler bug

Fix a bug that affects newer versions of libguestfs where the handler
is erroneously closed in disable_guestfs
Add missing `raise' keywords

There were cases where a FatalError exception was constructed but never
raised
windows: Use RIDs instead of Usernames

Relative IDs are safer. The Administrator's RID is always 500. The
name may change.
windows: Better handle shrink corner cases

Better handle the case where shrink querymax does not return any
reclaimable space.