armv7m: Forward init-svtor property to CPU object

Create an "init-svtor" property on the armv7m container
object which we can forward to the CPU object.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180220180325.29818-8-peter.maydell@linaro.org