KVM: PPC: Book3S HV: Only accept host PVR value for guest PVR

Since the guest can read the machine's PVR (Processor Version Register)
directly and see the real value, we should disallow userspace from
setting any value for the guest's PVR other than the real host value.
Therefore this makes kvm_arch_vcpu_set_sregs_hv() check the supplied
PVR value and return an error if it is different from the host value,
which has been put into vcpu->arch.pvr at vcpu creation time.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>