Fix: Set the stream class' id in the packet header

The stream id was used in the packet header whereas the
stream class' id should be used.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Implement CTF-IR event getters

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Tests: Add event getter tests

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>