iotests: Fix stuck NBD process on 33

iotests: Fix stuck NBD process on 33

Commit afe35cde6 added additional actions to test 33, but forgot
to reset the image between tests.  As a result, './check -nbd 33'
fails because the qemu-nbd process from the first half is still
occupying the port, preventing the second half from starting a
new qemu-nbd process.  Worse, the failure leaves a rogue qemu-nbd
process behind even after the test fails, which causes knock-on
failures to later tests that also want to start qemu-nbd.

Reported-by: Max Reitz <[email protected]>
Signed-off-by: Eric Blake <[email protected]>
Message-Id: <20180312211156[email protected]>
Reviewed-by: Max Reitz <[email protected]>