hw/sysbus: Introduce TYPE_DYNAMIC_SYS_BUS_DEVICE

hw/sysbus: Introduce TYPE_DYNAMIC_SYS_BUS_DEVICE

Some TYPE_SYS_BUS_DEVICEs can be optionally dynamically
plugged on the TYPE_PLATFORM_BUS_DEVICE.
Rather than sometimes noting that with comment around
the 'user_creatable = true' line in each DeviceRealize
handler, introduce an abstract TYPE_DYNAMIC_SYS_BUS_DEVICE
class.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alexander Graf <graf@amazon.com>
Reviewed-by: Clément Mathieu--Drif <clement.mathieu--drif@eviden.com>
Message-Id: <20250125181343.59151-4-philmd@linaro.org>