For vz containers, monotonic time freezes when a container is
checkpointed. This makes the zdtm posix timer test fail.

To fix this, obtain time passed, which is used to check timer results,
using the clock used by the timer.
Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: Stanislav Kinsbursky <skinsbursky@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>