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>