Comment 7 for bug 1533876

what we see is that when _set_device_mtu is called on the vhostuser device the device becomes non-functional:
[root@compute-0-0 ~]# ovs-vsctl show
f954b20d-8b91-4357-a6fa-df707f0bd1e6
    Bridge br-int
        fail_mode: secure
        Port "vhu272ad051-d5"
            Interface "vhu272ad051-d5"
                error: "could not open network device vhu272ad051-d5 (No such device)"

And its correspondent socket (/var/run/openvswitch) doesn't exist (==the call itself changed the state of the port for some reason)
Removing the call (temporarily for debugging) solved the issue.