qapi-events: add 'if' condition to implicit event enum

Add condition to QAPIEvent enum members based on the event 'if'.

The generated code remains unconditional for now. Later patches
generate the conditionals (also there is no additional coverage of
this change in qapi-schema-test.out since the event_names enum is an
implicit type created by qapi/events.py).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20181213123724.4866-11-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>