Support vhost-interface placement to specific workers

[Carsten Koester]

This is a feature request to support vhost-user placement on specific VPP workers (ref. "vhost thread" command in https://docs.fd.io/vpp/17.04/clicmd_src_vnet_devices_virtio.html or equivalent API call).

The objective, from a performance point of view, would be:

- to be able to completely separate DPDK workers (physical interfaces) from vhost-user interfaces in VPP. VPP startup.conf provides the ability to limit DPDK-input processes to a defined subset of VPP workers, so if we could limit vhost-user interfaces to a defined subset of VPP workers also, then we could completely isolate them. For this to work, all that would be required would be to statically specify a subset of workers (e.g. "workers 2, 3 and 4") and let networking-vpp round-robin or otherwise load-balance between these workers.

Ideally, there would be a way to re-balance interfaces to workers if VPP worker configuration is changed.

- in a more advanced approach, one could selectively assign interfaces on a port-by-port basis (and, for example, assign low-traffic interfaces such as management interfaces at a higher ports-per-worker ratio than interfaces known to carry high traffic load). This, however, would require networking-vpp to be be aware of the purpose of a specific port or network, and perhaps some proprietary tag or property to be associated with a port or network -- not sure how feasible that is.

[Ian Wells (ijw-ubuntu)]

The current post-18.07 VPP master now has the APIs we require for this (https://gerrit.fd.io/r/#/c/14445/).