KVM: x86 emulator: cleanup nop emulation

Make it more explicit what we are checking for.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/emulate.c

@@ -2799,8 +2799,8 @@ special_insn:
 			goto done;
 		break;
 	case 0x90: /* nop / xchg r8,rax */
-		if (!(c->rex_prefix & 1)) { /* nop */
-			c->dst.type = OP_NONE;
+		if (c->dst.ptr == (unsigned long *)&c->regs[VCPU_REGS_RAX]) {
+			c->dst.type = OP_NONE;  /* nop */
 			break;
 		}
 	case 0x91 ... 0x97: /* xchg reg,rax */