Add VIF_VHOSTUSER based on QEMU vhost-user feature for OpenStack Compute (nova)

Add VIF_VHOSTUSER based on the new vhost-user feature of QEMU/Libvirt. vhost-user is an efficient network I/O interface between a guest and a userspace vswitch. vhost-user has been developed to enable N x 10G Virtio-net workloads for OpenStack NFV deployments.