This new option is to have user to pass a fd
in place of statefile name.

Simple usage with file open in bash:
===
$rm -f *.log; lxc-execute -n foo -- pi1  44444
$rm -rf /tmp/sf; lxc-checkpoint -n foo -k --statefd 3 3>/tmp/sf && lxc-restart -n bar --statefd 4 4</tmp/sf
===
Signed-off-by: Michel Normand <normand@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>