[libvirt] Move esxVMX_IndexToDiskName to util.c

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. So, I made two patches for either option and ask you for comments. Matthias