hw/cxl/mailbox: Use new UUID network order define for cel_uuid

hw/cxl/mailbox: Use new UUID network order define for cel_uuid

The cel_uuid was programatically generated previously because there was
no static initializer for network order UUIDs.

Use the new network order initializer for cel_uuid.  Adjust
cxl_initialize_mailbox() because it can't fail now.

Update specification reference.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Gregory Price <gregory.price@memverge.com>
Tested-by: Gregory Price <gregory.price@memverge.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20230206172816.8201-11-Jonathan.Cameron@huawei.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>