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.