based on patch from: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>

but also:
* remove the deprecated --directory one.
* change liblxc api of checkpoint/restart to use fd and not string.
* explicitely report error messages for the checkpoint/restart stub functions.
Signed-off-by: Michel Normand <normand@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>