Delay before destroying loopback devices

Generally I don't approve of just adding an arbitrary delay to
avoid a problem but in this case I'm going to let it slide. We
may need to delay briefly after 'zpool destroy' returns to ensure
the loopback devices are closed. If they aren't closed than
losetup -d will not be able to destroy them. Unfortunately,
there's no easy state the check so we'll have to make due with
a simple delay.