qapi script: use same function to generate enum string

Prior to this patch, qapi-visit.py used custom code to generate enum
names used for handling a qapi union. Fix it to instead reuse common
code, with identical generated results, and allowing future updates to
generation to only need to touch one place.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>