Hello!

quoted

a) ovs_in_container does not send VHOST_USER_SET_MEM_TABLE

Please check if rte_eth_dev_start() is called.
(rte_eth_dev_start -> virtio_dev_start -> vtpci_reinit_complete -> kick_all_vq)

quoted

b) set_vring_addr() does not make sure that dev->mem is set.
 I am preparing a patch to fix (b).

Yes, it seems like a bug, lack of necessary check.

 I've made some progress about (a). It's tricky. This caused by this fragment:

        if (vhost_user_read(vhost->sockfd, &msg, len, fds, fd_num) < 0)
                return 0;

 Here you ignore errors. And this particular request for some reason ends up in EBADF. The most magic part is that sometimes it just
works...
 Not sure if i can finish it today, and here in Russia we have New Year holidays until 11th.

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia