ir: add trace UUID getter and setter

ir: add trace UUID getter and setter

The trace object's creation does not generate a UUID anymore: a new
trace starts with an unset UUID. When you create a CTF writer object, it
creates a trace object, generates a UUID, and set its trace's UUID. This
ensures backward compatibility with pre-2.0 CTF writer while allowing a
new trace object to have no UUID.

Tests are updated accordingly.

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