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

[Christophe Fergeau]

Hey,

I've been having issues recently after upgrading to a newer libvirt on a
RHEL6 box. 'virsh start vmname' fails with
'error :internal error Failed to get user record for name '107': No such
file or directory '
Looking deeper into this, this comes from 'user not found' errors from
getpwnam_r/getgrnam_r not being correctly handled. This patch series attempts
to improve that, but I'm not that happy with the seemingly random values of
errno being checked there.

Christophe