This provides a new struct that contains a buffer for the RPC
message header+payload, as well as a decoded copy of the message
header. There is an API for applying a XDR encoding & decoding
of the message headers and payloads. There are also APIs for
maintaining a simple FIFO queue of message instances.

Expected usage scenarios are:

To send a message

   msg = virNetMessageNew()

   ...fill in msg->header fields..
   virNetMessageEncodeHeader(msg)
   ...loook at msg->header fields to determine payload filter
   virNetMessageEncodePayload(msg, xdrfilter, data)
   ...send msg->bufferLength worth of data from buffer

To receive a message

   msg = virNetMessageNew()
   ...read VIR_NET_MESSAGE_LEN_MAX of data into buffer
   virNetMessageDecodeLength(msg)
   ...read msg->bufferLength-msg->bufferOffset of data into buffer
   virNetMessageDecodeHeader(msg)
   ...look at msg->header fields to determine payload filter
   virNetMessageDecodePayload(msg, xdrfilter, data)
   ...run payload processor

* src/Makefile.am: Add to libvirt-net-rpc.la
* src/rpc/virnetmessage.c, src/rpc/virnetmessage.h: Internal
  message handling API.
* testutils.c, testutils.h: Helper for printing binary differences
* virnetmessagetest.c: Validate all XDR encoding/decoding