qapi: Avoid unnecessary use of enum lookup table's sentinel

qapi: Avoid unnecessary use of enum lookup table's sentinel

Currently, the FOO_lookup[] generated for QAPI enum types are
terminated by a NULL sentinel.

A future patch will generate enums with "holes".  NULL-termination
will cease to work then.

To prepare for that, replace "have we reached the sentinel?"
predicates by "have we reached the FOO__MAX value?" predicates.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1503564371-26090-12-git-send-email-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>