[libvirt] Move esxVMX_IndexToDiskName to util.c

[pritesh]

> I needed the inverse function to virDiskNameToIndex() for the ESX
> driver and added it to esx_vmx.c. The pending VirtualBox 3.1 patch
> needs disk index to disk name mapping too. So I want to move
> esxVMX_IndexToDiskName() to util.c.
> 
> esxVMX_IndexToDiskName() handles indices up to 701. This limit comes
> from a gap in the disk name to disk index mapping of
> virDiskNameToIndex():
> 
> sdzy -> 700
> sdzz -> 701
> sdaaa -> 728
> sdaab -> 729
> 
> This line in virDiskNameToIndex() causes this gap:
> 
> idx = (idx + i) * 26;
> 
> It can be fixed by altering this line to:
> 
> idx = (idx + (i < 1 ? 0 : 1)) * 26;
> 
> But this change breaks compatibility for indices > 701.
> 

ACK to patch A.

I am not sure if we should go with patch B as it is good to squash bugs and 
not make them seem like a feature, I guess it is ok to break compatibility if 
it is real bug like this instead on inventing ways around it.

Regards,
Pritesh