qemu has 2 type of functions: shutdown and reboot. Shutdown
function has to be used for machine shutdown. Otherwise we cause
a reset with a bogus "cause" value, when we intended a shutdown.

Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20210625111842.3790-3-maxim.uvarov@linaro.org
[PMM: tweaked commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>