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: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index 57be9bf..1e368be 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -60,7 +60,7 @@
 
     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);