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

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