hw/arm/virt: Introduce a RAS machine option

hw/arm/virt: Introduce a RAS machine option

RAS Virtualization feature is not supported now, so
add a RAS machine option and disable it by default.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Dongjiu Geng <gengdongjiu@huawei.com>
Signed-off-by: Xiang Zheng <zhengxiang9@huawei.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 20200512030609.19593-3-gengdongjiu@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>