The newer "except <exception-type> as <exception>:" syntax is not
supported by Python 2.4, we need to use "except <exception-type>,
<exception>:".

Tested all trace backends with Python 2.4.
Reported-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: NLluís Vilanova <vilanova@ac.upc.edu>