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

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

Add two new APIs virNetServerNewPostExecRestart and
virNetServerPreExecRestart which allow a virNetServerPtr
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 all registered services
and clients

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