windows: Compute CurrentControlSet only ones

Add a current_control_set property in Registry class. This way the
CurrentControllSet registry key is only computed once.
Fix for PEP8's E265.

pep8 1.5 introduced a new test (E265) to ensure there's at least one
space before block comment.
Use @property to create the os attribute in Image

Use the @property decorator instead of the property() method to create a
read-only os attribute in Image class.
Add device attribute in Disk class

This attribute hosts the block device associated with this Disk
instance. This can be the the original media if the source media is a
block device, a loop device of the original media if the original media
is a regular file or a loop device of a temporary object if the original
media is a directory.