Use a virFreeCallback on virNetSocket to ensure safe release

Use a virFreeCallback on virNetSocket to ensure safe release

When unregistering an I/O callback from a virNetSocket object,
there is still a chance that an event may come in on the callback.
In this case it is possible that the virNetSocket might have been
freed already. Make use of a virFreeCallback when registering
the I/O callbacks and hold a reference for the entire time the
callback is set.

* src/rpc/virnetsocket.c: Register a free function for the
  file handle watch
* src/rpc/virnetsocket.h, src/rpc/virnetserverservice.c,
  src/rpc/virnetserverclient.c, src/rpc/virnetclient.c: Add
  a free function for the socket I/O watches