qom: Add check() argument to object_property_add_link()

There are currently three types of object_property_add_link() callers:

1. The link property may be set at any time.
2. The link property of a DeviceState instance may only be set before
   realize.
3. The link property may never be set, it is read-only.

Something similar can already be achieved with
object_property_add_str()'s set() argument.  Follow its example and add
a check() argument to object_property_add_link().

Also provide default check() functions for case #1 and #2.  Case #3 is
covered by passing a NULL function pointer.

Cc: Peter Crosthwaite <peter.crosthwaite@petalogix.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Anthony Liguori <aliguori@amazon.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
[AF: Tweaked documentation comment]
Signed-off-by: Andreas Färber <afaerber@suse.de>

ui/console.c

@@ -1181,6 +1181,7 @@ static QemuConsole *new_console(DisplayState *ds, console_type_t console_type)
     s = QEMU_CONSOLE(obj);
     object_property_add_link(obj, "device", TYPE_DEVICE,
                              (Object **)&s->device,
+                             object_property_allow_set_link,
                              OBJ_PROP_LINK_UNREF_ON_RELEASE,
                              &local_err);
     object_property_add_uint32_ptr(obj, "head",