KVM: arm64: Refuse to run VCPU if the PMU doesn't match the physical CPU

Userspace can assign a PMU to a VCPU with the KVM_ARM_VCPU_PMU_V3_SET_PMU
device ioctl. If the VCPU is scheduled on a physical CPU which has a
different PMU, the perf events needed to emulate a guest PMU won't be
scheduled in and the guest performance counters will stop counting. Treat
it as an userspace error and refuse to run the VCPU in this situation.

The VCPU is flagged as being scheduled on the wrong CPU in vcpu_load(), but
the flag is cleared when the KVM_RUN enters the non-preemptible section
instead of in vcpu_put(); this has been done on purpose so the error
condition is communicated as soon as possible to userspace, otherwise
vcpu_load() on the wrong CPU followed by a vcpu_put() could clear the flag.

Suggested-by: Marc Zyngier <maz@kernel.org>