ctf: rename bt_msg_iter::msg_iter to self_msg_iter

ctf: rename bt_msg_iter::msg_iter to self_msg_iter

This follows the naming convention that I have used so far.  It helps
knowing which kind of msg iter we are talking about, given that there
are multiple kinds.

Change-Id: Ifece52b06312b0c5f878313171ca4973643e305d
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2320
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>