Add JSON serialization of virNetServerServicePtr objects for process re-exec()

Add JSON serialization of virNetServerServicePtr objects for process re-exec()

Add two new APIs virNetServerServiceNewPostExecRestart and
virNetServerServicePreExecRestart which allow a virNetServerServicePtr
object to be created from a JSON object and saved to a
JSON object, for the purpose of re-exec'ing a process.

This includes serialization of the listening sockets associated
with the service

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