Add helper function and call it for each trace event group added.
Makes sure that events added at module load time are initialized
properly.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20210601132414.432430-6-kraxel@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>