Keep all MSA-related code altogether.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201120210844.2625602-4-f4bug@amsat.org>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>