trace-cmd: Add helper function get_instance_dir()
trace-cmd: Add helper function get_instance_dir()

Add the helper function get_instance_dir() that returns the path to
the instance directory.

Signed-off-by: Steven Rostedt <43232e92d70cc7aa53504ad0397085ee47bad87f@goodmis.org>

bigzz/trace-cmd,bigzz/trace-cmd