patches: Assign topic to all patches

Divide them into three groups:

  * debian - patches that are Debian-specific;

  * backport - patches that have been cherry-picked from a
    newer libvirt version and will naturally be dropped as soon
    as that version is imported into Debian;

  * forward - patches that are not taken from upstream but are
    not inherently Debian-specific either. These should be sent
    upstream so that we can eventually drop them from the Debian
    package.