migration/multifd: fix hangup with TLS-Multifd due to blocking handshake

The qemu main loop could hang up forever when we enable TLS+Multifd.
The Src multifd_send_0 invokes tls handshake, it sends hello to sever
and wait response.
However, the Dst main qemu loop has been waiting recvmsg() for multifd_recv_1.
Both of Src and Dst main qemu loop are blocking and waiting for reponse which
results in hanging up forever.

Src: (multifd_send_0)                                              Dst: (multifd_recv_1)
multifd_channel_connect                                            migration_channel_process_incoming
  multifd_tls_channel_connect                                        migration_tls_channel_process_incoming
    multifd_tls_channel_connect                                        qio_channel_tls_handshake_task
       qio_channel_tls_handshake                                         gnutls_handshake
          qio_channel_tls_handshake_task                                       ...
            qcrypto_tls_session_handshake                                      ...
              gnutls_handshake                                                 ...
                   ...                                                         ...
                recvmsg (Blocking I/O waiting for response)                recvmsg (Blocking I/O waiting for response)

Fix this by offloadinig handshake work to a background thread.

Reported-by: Yan Jin <jinyan12@huawei.com>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Chuan Zheng <zhengchuan@huawei.com>
Message-Id: <1604643893-8223-1-git-send-email-zhengchuan@huawei.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>