There is a problem for power button that it will not work if an early
system_powerdown request happens before guest gpio driver loads.

Fix this problem by using gpio_key.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Message-id: 1458221140-15232-3-git-send-email-zhaoshenglong@huawei.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>