hw/xen: Have legacy Xen backend inherit from DYNAMIC_SYS_BUS_DEVICE

hw/xen: Have legacy Xen backend inherit from DYNAMIC_SYS_BUS_DEVICE

Because the legacy Xen backend devices can optionally be plugged on the
TYPE_PLATFORM_BUS_DEVICE, have it inherit TYPE_DYNAMIC_SYS_BUS_DEVICE.
Remove the implicit TYPE_XENSYSDEV instance_size.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alexander Graf <graf@amazon.com>
Tested-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20250125181343.59151-10-philmd@linaro.org>