zdtm.py: Add step-by-step execution

When debugging it's useful to stop zdtm.py at certain points and
check what's going on out there. For this the --sbs (step-by-step)
option is introduced. It will make zdtm.py stop and wait for user
keypress before dump, before restore and after restore.

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