Trim changelogs to 2 years.
    
    Current changelogs take up several MBs, especially if you have
    all the libguestfs subpackages installed.
    
    Trim them to 2 years, which is double the Fedora support schedule, as
    suggested by Toshio Kuratomi
    (https://lists.fedoraproject.org/pipermail/devel/2013-April/181512.html).