glib-events: Allow zero timeouts for timer
glib-events: Allow zero timeouts for timer

In libvirt, it's perfectly possible and widely used to have disabled
timers (timeout=-1) and fire them up 'randomly' with timeout=0.
However, with current mapping into glib mainloop it's not possible
and causing troubles.

libvirt/libvirt-glib,JGulic/libvirt-glib,libvirt/libvirt-glib,JGulic/libvirt-glib,JGulic/libvirt-glib,JGulic/libvirt-glib,JGulic/libvirt-glib,libvirt/libvirt-glib