trace-cmd: Clear out instances as well
trace-cmd: Clear out instances as well

If an instance is specified on the command line, make sure to clear
out its trace buffer before tracing too.

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

bigzz/trace-cmd,bigzz/trace-cmd