trace: make trace_thread_create() use its function arg

This patch makes trace_thread_create() to use its function arg to
initialize thread.  The other choice is to make this a function to use
void arg, but i prefer this way.

Signed-off-by: Jun Koi <junkoi2004@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
