network-binding-plugin: add plugin for vhostuser interfaces.

[bgaussen]

What this PR does / why we need it:

This design proposal aims at implementing a new network binding plugin to support vhostuser interfaces. This will allow fast userspace datapath when used with a userspace dataplane like OVS-DPDK or VPP. This design proposal takes into consideration sockets sharing issues between kubevirt and dataplane pods.

Special notes for your reviewer:

Checklist

This checklist is not enforcing, but it's a reminder of items that could be relevant to every PR. Approvers are expected to review this list.

• [X] Design: A design document was considered and is present.
• [X] PR: The PR description is expressive enough and will help future contributors
• [X] Community: Announcement to kubevirt-dev was considered

Release note:

NONE