windows: Cleanup the VirtIO driver installation

Add a mechanism in OSBase to add cleanup jobs. Use this mechanism to
cleanup the VirtIO drivers installation.
Allow having hidden sysprep parameters

The sysprep parameters with the hidden attribute enabled are not
displayed to the user.
Add --install-virtio option in snf-mkimage

If this is enabled, the program will try to install the specified VirtIO
drivers to the image.
Add a wizard page for the VirtIO drivers

Add a page in the dialog wizard that displays the current state of
the VirtIO drivers of the image and allow the user to update the
installed VirtIO drivers. This page will only be shown if the media
hosts a Windows image.
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