Allow criu >= 1.3 in c/r test

criu version 1.3 has been tagged, which has the minimal set of patches to allow
checkpointing and restoring containers. lxc-test-checkpoint-restore is now
skipped on any version of criu lower than 1.3.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Exit on errors in restore()'s worker

If we just return here, we end up with two processes executing the caller's
code, which is not good.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
finalize handler in lxcapi_restore

We can also narrow the scope of this, since we only need it in the process that
is actually going to use it.

Reported-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>