netdev-dpdk: vhost: Fix txq enabling in the absence of notifications.

netdev-dpdk: vhost: Fix txq enabling in the absence of notifications.

According to QEMU documentation (docs/specs/vhost-user.txt) one queue
should be enabled initially. More queues are enabled dynamically, by
sending message VHOST_USER_SET_VRING_ENABLE.

Currently all queues in OVS disabled by default. This breaks above
specification. So, queue #0 should be enabled by default to support
QEMU versions less than 2.5 and fix probable issues if QEMU will not
send VHOST_USER_SET_VRING_ENABLE for queue #0 according to documentation.
Also this will fix currently broken vhost-cuse support in OVS.

Fixes: 585a5beaa2a4 ("netdev-dpdk: vhost-user: Fix sending packets to
                      queues not enabled by guest.")
Reported-by: Mauricio Vasquez B <mauricio.vasquezbernal@studenti.polito.it>
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>