veth: fix off by one error

0e869bf82f30ff6bce3d7cdc66779d8b642c82af introduces this bug, which chops
off the last character of the external veth name, and then subsequent
move_veth_to_bridge() calls fail:

(01.012478) Error (criu/net.c:1758): Can't get index of veth69A67O: No such device

travis-ci: success for veth: fix off by one error
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>