Adding support for multiple VIF drivers in KVM

- Building map of {trafficType, vifDriver} at configure time
- 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
- Also slight refactor of unit test code, and use jUnit 4 instead of 3, to match rest of codebase

Signed-off-by: Hugo Trippaers <htrippaers@schubergphilis.com>
CLOUDSTACK-1304 Set permissions using the chmod ant task