The virNetServerClient object had a hardcoded limit of 10 requests
per client. Extend constructor to allow it to be passed in as a
configurable variable. Wire this up to the 'max_client_requests'
config parameter in libvirtd

* daemon/libvirtd.c: Pass max_client_requests into services
* src/rpc/virnetserverservice.c, src/rpc/virnetserverservice.h: Pass
  nrequests_client_max to clients
* src/rpc/virnetserverclient.c, src/rpc/virnetserverclient.h: Allow
  configurable request limit

When a filter steals an RPC message, that message must
not be freed, except by the filter code itself

* src/rpc/virnetserverclient.c: Don't free stolen RPC
  messages

To facilitate creation of new daemons providing XDR RPC services,
pull a lot of the libvirtd daemon code into a set of reusable
objects.

 * virNetServer: A server contains one or more services which
   accept incoming clients. It maintains the list of active
   clients. It has a list of RPC programs which can be used
   by clients. When clients produce a complete RPC message,
   the server passes this onto the corresponding program for
   handling, and queues any response back with the client.

 * virNetServerClient: Encapsulates a single client connection.
   All I/O for the client is handled, reading & writing RPC
   messages.

 * virNetServerProgram: Handles processing and dispatch of
   RPC method calls for a single RPC (program,version).
   Multiple programs can be registered with the server.

 * virNetServerService: Encapsulates socket(s) listening for
   new connections. Each service listens on a single host/port,
   but may have multiple sockets if on a dual IPv4/6 host.

Each new daemon now merely has to define the list of RPC procedures
& their handlers. It does not need to deal with any network related
functionality at all.