tracetool: avoid str.rpartition() Python 2.5 function

tracetool: avoid str.rpartition() Python 2.5 function

The str.rpartition() function is related to str.split() and is used for
splitting strings.  It was introduced in Python 2.5 and therefore cannot
be used in tracetool as Python 2.4 compatibility is required.

Replace the code using str.rsplit().

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: Lluís Vilanova <vilanova@ac.upc.edu>