qom: Allow optional sugar props

Global properties have an @optional field, which allows to apply a given
property to a given type even if one of its subclasses doesn't support
it. This is especially used in the compat code when dealing with the
"disable-modern" and "disable-legacy" properties and the "virtio-pci"
type.

Allow object_register_sugar_prop() to set this field as well.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <159738953558.377274.16617742952571083440.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

softmmu/rtc.c

@@ -179,7 +179,8 @@ void configure_rtc(QemuOpts *opts)
         if (!strcmp(value, "slew")) {
             object_register_sugar_prop("mc146818rtc",
                                        "lost_tick_policy",
-                                       "slew");
+                                       "slew",
+                                       false);
         } else if (!strcmp(value, "none")) {
             /* discard is default */
         } else {