Adding support for multiple VIF drivers in KVM

- Step 1: Building map of {trafficType, vifDriver} at configure time
Adding support for multiple VIF drivers in KVM (commit 2)

- Use the relevant VIF driver for the given traffic type when call plug()
- Inform all vif drivers when call unplug(), as we no longer know traffic type
Refactor VIF driver choosing code and add unit tests

- Basic unit tests, just test default case
Added further unit tests

- Also slight refactor of unit test code, and use jUnit 4 instead of 3, to match rest of codebase