target-arm: fix a segmentation fault due to illegal memory access

target-arm: fix a segmentation fault due to illegal memory access

The elements of kvm_devices_head list are freed in kvm_arm_machine_init_done(),
but we still access these illegal memory in kvm_arm_devlistener_del().

This will cause segment fault when booting guest with MALLOC_PERTURB_=1.

Signed-off-by: Zheng Xiang <xiang.zheng@linaro.org>
Message-id: 20180619075821.9884-1-zhengxiang9@huawei.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>