qapi: Invoke exception superclass initializer

pylint recommends that every exception class should explicitly
invoke the superclass __init__, even though things seem to work
fine without it.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1443565276-4535-4-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
