iotests/151: ensure subprocesses are cleaned up

iotests/151: ensure subprocesses are cleaned up

The iotest 151 creates a bunch of subprocesses, with their stdout
connected to a pipe but never reads any data from them and does
not gurantee the processes are killed on cleanup.

This triggers resource leak warnings from python when the
subprocess.Popen object is garbage collected.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>