c/r: use --restore-sibling option in CRIU

c/r: use --restore-sibling option in CRIU

After looking through some logs, it is a little cleaner to do it as
below, instead of what I originally posted.

Tycho

In order for LXC to be the parent of the restored process, CRIU needs to
restore init as its sibling, not as its child. This was previously accomplished
essentially via luck :). CRIU now has a --restore-sibling option which forces
this behavior that LXC expects. See more discussion in this thread:
http://lists.openvz.org/pipermail/criu/2014-September/thread.html#16330

v2: don't pass --restore-sibling to dump. This is mostly cosmetic, but will
    look less confusing in the logs if people ever look at them.

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