rpc: avoid crashing in pre-exec if no workers are present
    
    If max_workers is set to zero, then the worker thread pool won't be
    created, so when serializing state for pre-exec we must set various
    parameters to zero.
    
    Reviewed-by: John Ferlan <jferlan@redhat.com>
    Reviewed-by: Jim Fehlig <jfehlig@suse.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>