KVM: arm/arm64: check power_off in critical section before VCPU run

In case a vcpu off PSCI call is called just after we executed the
vcpu_sleep check, we can enter the guest although power_off
is set. Let's check the power_off state in the critical section,
just before entering the guest.

Signed-off-by: Eric Auger <eric.auger@linaro.org>
Reported-by: Christoffer Dall <christoffer.dall@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>