qapi-visit.py: Clean up a sloppy use of field prefix

generate_visit_struct_fields() generates the base type's struct member
name both with and without the field prefix.  Harmless, because the
field prefix is always empty there: only unboxed complex members have
a prefix, and those can't have a base type.

Clean it up anyway.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

scripts/qapi-visit.py

@@ -60,7 +60,7 @@ static void visit_type_%(full_name)s_fields(Visitor *m, %(name)s ** obj, Error *
 
     if base:
         ret += mcgen('''
-visit_start_implicit_struct(m, (void**) &(*obj)->%(c_name)s, sizeof(%(type)s), &err);
+visit_start_implicit_struct(m, (void**) &(*obj)->%(c_prefix)s%(c_name)s, sizeof(%(type)s), &err);
 if (!err) {
     visit_type_%(type)s_fields(m, &(*obj)->%(c_prefix)s%(c_name)s, &err);
     error_propagate(errp, err);