hw/xen: Prefer QOM cast for XenLegacyDevice

Makes the code less sensitive regarding changes in the class hierarchy which
will be performed in the next patch.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250127094129.15941-1-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
3 files changed