The release callback is called during object_property_del_all(), on a
live instance. But class properties are common among all
instances. It is not currently called, because we don't release
classes, but it would not be correct if we did.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200110153039.1379601-12-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>