src: ensure GSource background unref happens in correct event loop

src: ensure GSource background unref happens in correct event loop

The g_idle_add function adds a callback to the primary GMainContext.

To workaround the GSource unref bugs, we need to add our callbacks
to the GMainContext that is associated with the GSource being
unref'd. Thus code using the per-VM virEventThread must use its
private GMainContext.

Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>