hw/i386: Have X86_IOMMU devices inherit from DYNAMIC_SYS_BUS_DEVICE

hw/i386: Have X86_IOMMU devices inherit from DYNAMIC_SYS_BUS_DEVICE

Do not explain why _X86_IOMMU devices are user_creatable,
have them inherit TYPE_DYNAMIC_SYS_BUS_DEVICE, to explicit
they can optionally be plugged on TYPE_PLATFORM_BUS_DEVICE.

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: <20250125181343.59151-7-philmd@linaro.org>