python: KeyError shouldn't be raised from __getattr__

python: KeyError shouldn't be raised from __getattr__

On Python 3 hasattr only intercepts AttributeError exception.
On Python2, instead, hasattr intercepts all the exceptions.

This means __getattr__ shouldn't return KeyError when the attribute
doesn't exists, but it should raise AttributeError instead.

Fixes: 2d54d8011e14 ("Python-IDL: getattr after mutate fix")
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Russell Bryant <russell@ovn.org>