KVM: x86: Wrong operand size for far ret

Indeed, Intel SDM specifically states that for the RET instruction "In 64-bit
mode, the default operation size of this instruction is the stack-address size,
i.e. 64 bits."

However, experiments show this is not the case. Here is for example objdump of
small 64-bit asm:

  4004f1:	ca 14 00             	lret   $0x14
  4004f4:	48 cb                	lretq
  4004f6:	48 ca 14 00          	lretq  $0x14

Therefore, remove the Stack flag from far-ret instructions.
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -4062,8 +4062,8 @@ static const struct opcode opcode_table[256] = {
 	G(ByteOp, group11), G(0, group11),
 	/* 0xC8 - 0xCF */
 	I(Stack | SrcImmU16 | Src2ImmByte, em_enter), I(Stack, em_leave),
-	I(ImplicitOps | Stack | SrcImmU16, em_ret_far_imm),
-	I(ImplicitOps | Stack, em_ret_far),
+	I(ImplicitOps | SrcImmU16, em_ret_far_imm),
+	I(ImplicitOps, em_ret_far),
 	D(ImplicitOps), DI(SrcImmByte, intn),
 	D(ImplicitOps | No64), II(ImplicitOps, em_iret, iret),
 	/* 0xD0 - 0xD7 */