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>