[libvirt] [PATCHv2] util: Fix virGet{User, Group}IDByName when the user/group cannot be found

[Christophe Fergeau]

Here is the v2 of this patch series.

Changes since v1:
- return an error only for documented error values returnef from
  getpwnam_r/getgrnam_r instead of trying to guess what error values
  mean 'entry not found'
- this addresses the indenting comment raised by Osier