target-arm: kvm: Differentiate registers based on write-back levels

Some registers like the CNTVCT register should only be written to the
kernel as part of machine initialization or on vmload operations, but
never during runtime, as this can potentially make time go backwards or
create inconsistent time observations between VCPUs.

Introduce a list of registers that should not be written back at runtime
and check this list on syncing the register state to the KVM state.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Message-id: 1437046488-10773-1-git-send-email-christoffer.dall@linaro.org
[PMM: tweaked a few comments, added the new argument to the stub
 write_list_to_kvmstate() in target-arm/kvm-stub.c]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
