ctf: only reset state of `bt_msg_iter` on _reset()

The `bt_msg_iter` configuration should be left untouched by the reset.

The `bt_msg_iter_reset()` is used at the creation of the `bt_msg_iter`
(`bt_msg_iter_create()`) and when seeking with it
(`bt_msg_iter_seek()`). It's also used by `src.ctf.fs` message iterators
when seeking is done.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I2ab948858aa5b955a3063174c9887c54d98bad5d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1416
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>