events: don't create glib IO watch for disabled handles
It's possible to create a handle to watch for file events which do not
watch for any file event. Such a handle can be enabled later with
virt_viewer_events_update_handle() by setting some conditions to watch for.
When a handle is disabled after it has been created,
virt_viewer_events_update_handle() makes sure it removes the corresponding
virt_viewer_events_handle::source IO watch if any was set.
virt_viewer_events_add_handle() will always create a
virt_viewer_events_handle::source IO watch even if the handle is not
watching for any events.
This commit makes consistent by only creating a watch with g_io_add_watch()
when the caller asked to watch for some events.
Based on commit d71c143936a35cd6c3f23ae0cbf7f3215d944051 from
libvirt-glib.
Original author: Christophe Fergeau <cfergeau@redhat.com>
Related to: rhbz#1243228