[Libguestfs] hivex: Make node names and value names with embedded null characters accessible

On Windows, there exist at least two APIs for dealing with the Registry: The Win32 API (RegCreateKeyA, RegCreateKeyW, etc.) works with null-terminated ASCII or UTF-16 strings. The native API (ZwCreateKey, etc.), on the other hand works with UTF-16 strings that are stored as buffers+length and may contain null characters. Malware authors have been relying on the Win32 API's inability to properly work with such names for several years. These changes make such names accessible from hivex.