scripts/qapi.py: Avoid syntax not supported by Python 2.4

The Python "except Foo as x" syntax was only introduced in
Python 2.6, but we aim to support Python 2.4 and later.
Use the old-style "except Foo, x" syntax instead, thus
fixing configure/compile on systems with older Python.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>