ir: stream: add bt_ctf_stream_is_writer()

This function indicates whether or not the stream is in
"CTF writer mode", that is, its trace parent was created by a
CTF writer object and it has an open file descriptor.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>