hw/intc/arm_gicv3_its: Fix the VM termination in vm_change_state_handler()

hw/intc/arm_gicv3_its: Fix the VM termination in vm_change_state_handler()

The commit cddafd8f353d ("hw/intc/arm_gicv3_its: Implement state save
/restore") breaks the backward compatibility with the older kernels
where vITS save/restore support is not available. The vmstate function
vm_change_state_handler() should not be registered if the running kernel
doesn't support ITS save/restore feature. Otherwise VM instance will be
killed whenever vmstate callback function is invoked.

Observed a virtual machine shutdown with QEMU-2.10+linux-4.11 when testing
the reboot command "virsh reboot <domain> --mode acpi" instead of reboot.

KVM Error: 'KVM_SET_DEVICE_ATTR failed: Group 4 attr 0x00000000000001'

Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-id: 1509712671-16299-1-git-send-email-shankerd@codeaurora.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>