src.ctf.fs: make create_streams_for_trace use bt2::StreamClass to instantiate streams

src.ctf.fs: make create_streams_for_trace use bt2::StreamClass to instantiate streams

Wrap the stream class (ds_file_group->sc->ir_sc) in a bt2::StreamClass,
then use that to instantiate streams.

Change-Id: I9e9917dd96b0d1115b443463a0a2cd8f2da42323
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8437
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12362
Tested-by: jenkins <jenkins@lttng.org>