The unified hierarchy controller doesn't have any name, so criu hangs
while parsing this sort of strings.

Let's skip those for now, until we implement cgroup2 support.

https://github.com/xemul/criu/issues/252

v2: Code comments
Reported-and-tested-by: Adrian Reber <adrian@lisas.de>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>