[libvirt] [libvirt-glib 0/4] Add API to build <timer name="hpet" present="no"/>

This series allows to generate <timer name="hpet" present="no"/> using libvirt-gconfig. This is a prerequisite for fixing this Boxes bug https://bugzilla.gnome.org/show_bug.cgi?id=722293 Patch 2/4 doesn't anything to do in this series, but this was easy to implement after 1/4 so I slipped it there. Christophe