Fedora 18 changes the way how to work with services in spec files. It introduces new macros - %systemd_post, %systemd_preun and %systemd_postun; which replace scriptlets from Fedora 17 and older (see https://fedoraproject.org/wiki/Packaging:ScriptletSnippets#Systemd, https://bugzilla.redhat.com/show_bug.cgi?id=850016).
I pushed the following to Rawhide: http://pkgs.fedoraproject.org/cgit/libguestfs.git/commit/?id=b6d75ce6689d37797709ce115070dd8ba7aee629 It will get merged into f18 shortly.