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.

Backports commit 9d3524b39e1fe5f3bb7a990ad7841e469e954a3b from qemu

qemu/scripts/qapi-visit.py

@@ -294,7 +294,8 @@ 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);