CRIU has added support for passing --cgroup-root on dump, which we should
use (see the criu commit 07d259f365f224b32914de26ea0fd59fc6db0001 for
details). Note that we don't have to do any version checking or anything,
because CRIU just ignored --cgroup-root on checkpoint before, so passing it
is safe, and will result in correct behavior when a sufficient version of
CRIU is present.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>