netdev-dpdk: add dpdk vhost-user ports

netdev-dpdk: add dpdk vhost-user ports

This patch adds support for a new port type to the userspace
datapath called dpdkvhostuser.

A new dpdkvhostuser port will create a unix domain socket which
when provided to QEMU is used to facilitate communication between
the virtio-net device on the VM and the OVS port on the host.

vhost-cuse ('dpdkvhost') ports are still available as 'dpdkvhostcuse'
ports and will be enabled if vhost-cuse support is detected in the
DPDK build specified during compilation of the switch. Otherwise,
vhost-user ports are enabled.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>