Add support for XMLb files

While reviewing non-reproducible issues with the live-image images Wiki, I've noticed that appstream generates binary versions of XML files with libxmlb. After the first 8 bytes, a random/timestamp-based value of 16 bytes is written. See also Jenkins

Neither file nor diffoscope recognises this type of file yet and the difference is shown as a binary dump.

These files can be found on a regular GNOME or KDE installation or when appstream is additionally installed. They are located in /var/cache/app-info/cache and have the extension xb