windows: Compute CurrentControlSet only ones

Add a current_control_set property in Registry class. This way the
CurrentControllSet registry key is only computed once.
Remove Sysprep Params Wizard Page

Now that all sysprep parameters are optional, wizard page is no longer
needed.
Add extra virtio sysprep parameter in windows

This parameter points to a directory that hosts virtio drivers for
windows.
Extend sysprep params to support a value check

Extend the mechanism to support passing a function to the syspep
parameter which can be used to check if the supplied value is correct.
This is used in virtio sysprep param to check if the needed files are
hosted in the directory the user supplied.
windows: Install viostor driver if missing

Add a dirty way to install the viostor driver in windows in order
to make them bootable. After booting we need to reinstall it in a
proper way. This works on Windows 2008R2
windows: Clean up Registry node_set_value code

In the Registry class whenever a node value is updated, make sure the
helper methods sz(), dword() and binary() are used