hw/xen: Constify xenstore_be::XenDevOps

XenDevOps @ops is not updated, mark it const.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Paul Durrant <paul@xen.org>
Message-Id: <20240510104908.76908-4-philmd@linaro.org>
2 files changed