Add two new APIs virNetServerClientNewPostExecRestart and
virNetServerClientPreExecRestart which allow a virNetServerClientPtr
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 connected socket associated
with the client
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>