qdev: release parent properties on dc->init failure

While looking into hot-plugging bridges, I can create a qemu segfault via:

$ device_add pci-bridge

Bridge chassis not specified. Each bridge is required to be assigned a unique chassis id > 0.
**
ERROR:qom/object.c:389:object_delete: assertion failed: (obj->ref == 0)

I'm proposing to fix this by adding a call to 'object_unparent()', before the
call to qdev_free(). I see there is already a precedent for this usage pattern as
seen in qdev_simple_unplug_cb():

/* can be used as ->unplug() callback for the simple cases */
int qdev_simple_unplug_cb(DeviceState *dev)
{
    /* just zap it */
    object_unparent(OBJECT(dev));
    qdev_free(dev);
    return 0;
}

Signed-off-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>