KVM: x86: NOP emulation clears (incorrectly) the high 32-bits of RAX

On long-mode the current NOP (0x90) emulation still writes back to RAX.  As a
result, EAX is zero-extended and the high 32-bits of RAX are cleared.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -4688,8 +4688,9 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 		break;
 	case 0x90 ... 0x97: /* nop / xchg reg, rax */
 		if (ctxt->dst.addr.reg == reg_rmw(ctxt, VCPU_REGS_RAX))
-			break;
-		rc = em_xchg(ctxt);
+			ctxt->dst.type = OP_NONE;
+		else
+			rc = em_xchg(ctxt);
 		break;
 	case 0x98: /* cbw/cwde/cdqe */
 		switch (ctxt->op_bytes) {