ctf: msg-iter: extract func to decode packet until specific states

ctf: msg-iter: extract func to decode packet until specific states

This now generic function will be used by future commits needing to
decode packet until after the first or last event of a packet.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I34d86c078e2e512c5491eed978ecc4cc8adcb3a8
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1431
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>