Port virtual-interfaces plugin to v2.1(v3) API

Ports virtual-interfaces extension and adapts it to the
v2.1/v3 API framework. API behaviour is identical.

- unittest code modified to share testing with both v2/v2.1
- Adds expected error decorators for API methods

Partially implements blueprint v2-on-v3-api

Change-Id: Ic25ae83ecfaaa461b6ddd044e04af844d02dd6b7
Merge "Port virtual-interfaces plugin to v2.1(v3) API"