rpc: Initialize a worker pool for max_workers=0 as well

rpc: Initialize a worker pool for max_workers=0 as well

Semantically, there is no difference between an uninitialized worker
pool and an initialized worker pool with zero workers. Let's allow the
worker pool to be initialized for max_workers=0 as well then which
makes the API more symmetric and simplifies code. Validity of the
worker pool is delegated to virThreadPoolGetMaxWorkers instead.

This patch fixes segmentation faults in
virNetServerGetThreadPoolParameters and
virNetServerSetThreadPoolParameters for the case when no worker pool
is actually initialized (max_workers=0).

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>