`ctf` plugin: `bt_msg_iter`: use BT_COMP_LOG*() instead of BT_LOG*()

`ctf` plugin: `bt_msg_iter`: use BT_COMP_LOG*() instead of BT_LOG*()

As of this patch, the users of `bt_msg_iter` set the new self component
parameter to `NULL` as this work is reserved for a following patch.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I98328d43e0f67d1c4b1cf5493fa9c9cd02a32393
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1498
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>