Defines the basics of a generic RPC protocol in XDR

Defines the basics of a generic RPC protocol in XDR

This patch defines the basics of a generic RPC protocol in XDR.
This is wire ABI compatible with the original remote_protocol.x.
It takes everything except for the RPC calls / events from that
protocol

 - The basic header virNetMessageHeader (aka remote_message_header)
 - The error object virNetMessageError  (aka remote_error)
 - Two dummy objects virNetMessageDomain & virNetMessageNetwork
   sadly needed to keep virNetMessageError ABI compatible with
   the old remote_error

The RPC protocol supports method calls, async events and
bidirectional data streams as before

* src/Makefile.am: Add rules for generating RPC code from
  protocol & define a new libvirt-net-rpc.la helper library
* src/rpc/virnetprotocol.x: New generic RPC protocol