img: Use relative names for image parents

As per criu bug 2871 absolute paths for parents are
incompatible with mount namespaces restore. That's OK
for p.haul, we can use relative.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
service: Don't kill exited page server

When we end the iteration, page server should have
exited. No need to kill it one more time (all the
more so, the pid could have been reused).

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>