inspector: Read information about Windows guests from the Registry.

inspector: Read information about Windows guests from the Registry.

This commit changes substantially the way that we get information
about Windows guests.  We now use the Windows Registry to get
information such as the version, product name and much else.

This uses Win::Hivex (ie. the hivex library).  'reged' is no longer
needed or used.

As an incidental change, this also tries harder to search for
%systemroot%, in the case when we cannot find boot.ini (ie.
Windows Vista and more recent).  This ensures we can get more
detail from those versions of Windows.