Extends vhostuser vif type in libvirt to support ovs-fp and lb-fp

Fast path technology is an user-space stack for high performance packet
offloading Linux kernel scenarios including acceleration of ip routing, linux
bridge, OVS, iptables, conntrack... Fast path is deployed on the compute nodes
to offload the packet processing. It can be configured with standard linux
commands (i.e. iproute2, ovs-vsctl, ...), allowing to support Neutron Mechanism
Driver/Agents based on Linux bridge or openvswitch.

To benefit from the offloading, a VM needs to use vhost-user instead of
vhost-net backend for virtio interfaces.

As a consequence, this blueprint proposes to extends the vhost-user support in
libvirt to support plug/unplug for the ovs-fp and lb-fp Mechanism Driver.

https://github.com/openstack/networking-6wind/blob/master/networking_6wind/ml2_drivers/openvswitch/mech_driver/mech_ovs_fp.py

https://github.com/openstack/networking-6wind/blob/master/networking_6wind/ml2_drivers/linuxbridge/mech_driver/mech_lb_fp.py