ALSA: intel8x0: Improve comments for VM optimization

The recently merged 228cf79376 looks a bit hackish while it is not.
The change was quite simple. In a virtualized environment the
patch unhacks old kludge introduced for old broken AC97 hardware.

This patch adds proper comment to "unkludge" code.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Konstantin Ozerkov <kozerkov@parallels.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/intel8x0.c

@@ -1077,6 +1077,13 @@ static snd_pcm_uframes_t snd_intel8x0_pcm_pointer(struct snd_pcm_substream *subs
 		}
 		if (civ != igetbyte(chip, ichdev->reg_offset + ICH_REG_OFF_CIV))
 			continue;
+
+		/* IO read operation is very expensive inside virtual machine
+		 * as it is emulated. The probability that subsequent PICB read
+		 * will return different result is high enough to loop till
+		 * timeout here.
+		 * Same CIV is strict enough condition to be sure that PICB
+		 * is valid inside VM on emulated card. */
 		if (chip->inside_vm)
 			break;
 		if (ptr1 == igetword(chip, ichdev->reg_offset + ichdev->roff_picb))