Add virIndexToDiskName and fix mapping gap

Add virIndexToDiskName and fix mapping gap

esxVMX_IndexToDiskName handles indices up to 701. This limit comes
from a mapping gap in virDiskNameToIndex:

  sdzy -> 700
  sdzz -> 701
  sdaaa -> 728
  sdaab -> 729

This line in virDiskNameToIndex causes this gap:

  idx = (idx + i) * 26;

Fixing it by altering this line to:

  idx = (idx + (i < 1 ? 0 : 1)) * 26;

Also add a new version of virIndexToDiskName that handles the inverse
mapping for arbitrary indices.

* src/esx/esx_vmx.[ch]: remove esxVMX_IndexToDiskName
* src/util/util.[ch]: add virIndexToDiskName and fix mapping gap
* tests/esxutilstest.c: update test to verify that the gap is fixed