In one exit path we forgot to free the virNetMessage object causing
a large memory leak for streams which send a lot of data. Some other
paths were calling VIR_FREE directly instead of virNetMessageFree
although this was (currently) harmless.

* src/rpc/virnetclientstream.c: Fix leak of msg object
* src/rpc/virnetclientprogram.c: Call virNetMessageFree instead
  of VIR_FREE

To facilitate creation of new clients using XDR RPC services,
pull alot of the remote driver code into a set of reusable
objects.

 - virNetClient: Encapsulates a socket connection to a
   remote RPC server. Handles all the network I/O for
   reading/writing RPC messages. Delegates RPC encoding
   and decoding to the registered programs

 - virNetClientProgram: Handles processing and dispatch
   of RPC messages for a single RPC (program,version).
   A program can register to receive async events
   from a client

 - virNetClientStream: Handles generic I/O stream
   integration to RPC layer

Each new client program now merely needs to define the list of
RPC procedures & events it wants and their handlers. It does
not need to deal with any of the network I/O functionality at
all.