The RPC server classes are extended to allow FDs to be received
from clients with calls. There is not currently any way for a
procedure to pass FDs back to the client with replies

* daemon/remote.c, src/rpc/gendispatch.pl: Change virNetMessageHeaderPtr
  param to virNetMessagePtr in dispatcher impls
* src/rpc/virnetserver.c, src/rpc/virnetserverclient.c,
  src/rpc/virnetserverprogram.c, src/rpc/virnetserverprogram.h:
  Extend to support FD passing