qemu-nbd: Switch to qemu_set_fd_handler Achieved by: - Remembering the server fd with a global variable, in order to access it from nbd_client_closed. - Checking nbd_can_accept() and updating server_fd handler whenever client connects or disconnects. Signed-off-by: Fam Zheng <famz@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Message-Id: <1432032670-15124-3-git-send-email-famz@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>