Cleanup: msg-iter.c: make `create_msg_*()` return msg

Cleanup: msg-iter.c: make `create_msg_*()` return msg

Those functions have a void return type but are returning the created
message using an output parameter.

Make them return the message instead.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Iff60a9436818f38d5b5046b921c0d53cdc1f2c66
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2307
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>