Fix sending/receiving of FDs when stream returns EAGAIN

Fix sending/receiving of FDs when stream returns EAGAIN

The code calling sendfd/recvfd was mistakenly assuming those
calls would never block. They can in fact return EAGAIN and
this is causing us to drop the client connection when blocking
ocurrs while sending/receiving FDs.

Fixing this is a little hairy on the incoming side, since at
the point where we see the EAGAIN, we already thought we had
finished receiving all data for the packet. So we play a little
trick to reset bufferOffset again and go back into polling for
more data.

* src/rpc/virnetsocket.c, src/rpc/virnetsocket.h: Update
  virNetSocketSendFD/RecvFD to return 0 on EAGAIN, or 1
  on success
* src/rpc/virnetclient.c: Move decoding of header & fds
  out of virNetClientCallDispatch and into virNetClientIOHandleInput.
  Handling blocking when sending/receiving FDs
* src/rpc/virnetmessage.h: Add a 'donefds' field to track
  how many FDs we've sent / received
* src/rpc/virnetserverclient.c: Handling blocking when
  sending/receiving FDs