With this patch, it's possible to shut down guests in parallel. Parallel
startup was possible before, but this functionality was not documented
properly.

To enable parallel startup set the START_DELAY to 0.

Parallel shutdown has a configurable parameter PARALLEL_SHUTDOWN that
defines the number of machines being shut down in parallel. Enabling
this feature changes the semantics of SHUTDOWN_TIMEOUT parameter that is
applied as a cumulative timeout to shutdown all guests on a URI.