hw/arm/virt: Fix corruption due to double free

As 4de9a88(hw/arm/virt: Fix memory leak reported by Coverity)
and 6e05a12(arm: fix memory leak) both handle the memory leak
reported by Coverity, this cause qemu corruption due to
double free.

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Message-id: 1427944026-8968-1-git-send-email-zhaoshenglong@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
