Fix: src.ctf.lttng-live: no stream beginning/end messages

Fix: src.ctf.lttng-live: no stream beginning/end messages

Issue
=====
`src.ctf.lttng-live` components don't emit stream beginning and end
messages. This is due to the `bt_msg_iter::emit_stream_begin_msg` field
being set to false at allocation time.

Solution
========
Set it to true right after the allocation.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Iae6b174e10070b32a877bb9ba7c1e637491ee15d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2022
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>