windows: split do_sysprep to reduce complexity

Add a new _exec_sysprep_tasks method that is called by do_sysprep and
hosts the actual code for executing the system preparation tasks
windows: Create a new registry module

This modules hosts the Registry class which contains all the registry
manipulation code previously hosted under the Windows class
Fix a bug in sysprep param handling in dialog_menu

If the needed sysprep parameters had all default values,
snf-image-creator would crash when trying to run the image
customization. This bug could not be triggered from the current os_type
classes.
windows: Reset the password when customizing

Don't ask the user to enter an Administrator password before cleaning up
a Windows image. Reset the password before starting the process and
assign the old value if needed at the end.
windows: Detect if the media is already sysprepped

Add code to check if the source media is a sysprepped windows image. If
so, don't allow it to be further customized.

This resolves #6
Rewrite the sysprep parameter mechanism

Rewrite the mechanism to have better control on the system preparation
parameters. Also change the way the parameters are updated in the UI.
Rewrite file selection function

Make it more general. Till know it was only used for selecting the input
media but other parts of the program should also make use of it.
Check for Winexe when customizing

We used to check for the presence of the winexe executable in the
Windows __init__ function. This is not optimal. The winexe check
should be done during customization. The user should be able to
dump, upload or register an image without having Winexe installed.
windows: Check if VirtIO drivers are installed

If the virtio drivers are not installed on the media, raise a FatalError