Convert armsse_realize() to use ERRP_GUARD(), following
the rules in include/qapi/error.h.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210510190844.17799-5-peter.maydell@linaro.org