glib crash via eventtest.c

I attempted to review some patches on Friday and started hitting occasional crashes via eventtest.c. Long story short it's a glib bug: https://gitlab.gnome.org/GNOME/glib/merge_requests/1358 It's a ref counting issue caused when g_source_remove (virEventRemoveHandle) is called from one thread, while the main loop is in a particular state in a different thread. The way vireventglib is implemented means every user initiated g_source_remove is likely called from a different thread so we risk hitting this. Not sure how likely it is in realworld usage, vireventtest.c is pretty pathologic in this area. We could change vireventglib.c to do the final source_unref from the idle callback which would avoid the problem Thanks, Cole