x86/kvm: virt_xxx memory barriers instead of mandatory barriers

virt_xxx memory barriers are implemented trivially using the low-level
__smp_xxx macros, __smp_xxx is equal to a compiler barrier for strong
TSO memory model, however, mandatory barriers will unconditional add
memory barriers, this patch replaces the rmb() in kvm_steal_clock() by
virt_rmb().

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kernel/kvm.c

@@ -396,9 +396,9 @@ static u64 kvm_steal_clock(int cpu)
 	src = &per_cpu(steal_time, cpu);
 	do {
 		version = src->version;
-		rmb();
+		virt_rmb();
 		steal = src->steal;
-		rmb();
+		virt_rmb();
 	} while ((version & 1) || (version != src->version));
 
 	return steal;