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

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