zdtm/stopped: fix stopped test

The main problem with the
test that it's never 'fail'.
Test can only 'pass' or hangs.

Signed-off-by: Konstantin Neumoin <kneumoin@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>