`ctf` plugin: `bt_msg_iter`: use object's log level

`ctf` plugin: `bt_msg_iter`: use object's log level

As of this patch, the users of `bt_msg_iter` set the new log level
parameter to their own (global) log level.

The goal is to eventually have a per-component log level for the `ctf`
plugin's component classes.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I279d22831c5857dc10622e6cabacf64c32fa5ed4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1488
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>