Refactor RPC client private data setup

Refactor RPC client private data setup

Currently there is a hook function that is invoked when a
new client connection comes in, which allows an app to
setup private data. This setup will make it difficult to
serialize client state during process re-exec(). Change to
a model where the app registers a callback when creating
the virNetServerPtr instance, which is used to allocate
the client private data immediately during virNetClientPtr
construction.

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