[Qemu-block] NBD handshake may block qemu main thread when socket delays or has packet loss

[Fangyi (C)]

    As we all know, socket is in blocking mode when nbd is negotiating 
with the other end. If the network is poor because of delay or packet 
loss, socket read or write will return very slowly. The mainloop events 
won't be handled in time util nbd handshake ends.

    Any advices to solve the problem?
    Thanks,

Regards,
    FANG Yi.