target/arm: Don't migrate CPUARMState.features

As feature flags are added or removed, the meanings of bits in the
`features` field can change between QEMU versions, causing migration
failures. Additionally, migrating the field is not useful because it is
a constant function of the CPU being used.

Fixes: LP:1914696
Signed-off-by: Aaron Lindsay <aaron@os.amperecomputing.com>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Tested-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/machine.c

@@ -810,7 +810,7 @@ const VMStateDescription vmstate_arm_cpu = {
         VMSTATE_UINT64(env.exclusive_addr, ARMCPU),
         VMSTATE_UINT64(env.exclusive_val, ARMCPU),
         VMSTATE_UINT64(env.exclusive_high, ARMCPU),
-        VMSTATE_UINT64(env.features, ARMCPU),
+        VMSTATE_UNUSED(sizeof(uint64_t)),
         VMSTATE_UINT32(env.exception.syndrome, ARMCPU),
         VMSTATE_UINT32(env.exception.fsr, ARMCPU),
         VMSTATE_UINT64(env.exception.vaddress, ARMCPU),