qapi-visit: Honor prefix of discriminator enum

When we added support for a user-specified prefix for an enum
type (commit 351d36e), we forgot to teach the qapi-visit code
to honor that prefix in the case of using a prefixed enum as
the discriminator for a flat union.  While there is still some
on-list debate on whether we want to keep prefixes, we should
at least make it work as long as it is still part of the code
base.

Reported-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1455665965-27638-1-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi-visit.py

@@ -293,7 +293,8 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
     case %(case)s:
 ''',
                      case=c_enum_const(variants.tag_member.type.name,
-                                       var.name))
+                                       var.name,
+                                       variants.tag_member.type.prefix))
         if simple_union_type:
             ret += mcgen('''
         visit_type_%(c_type)s(v, "data", &(*obj)->u.%(c_name)s, &err);