Implement lookup for podvif driver by vif

New pod-vif drivers, such as sriov and nested dpdk require their own implementation of active_vif & release_vif. In this situation VIFHandler should find it by vif (vif is incoming parameter), it already did it in _drv_for_vif, but for SRIOV (https://review.openstack.org/#/c/512280/27/kuryr_kubernetes/controller/handlers/vif.py) and for netsted dpdk (https://github.com/pliurh/kuryr-kubernetes/commit/fdbcf5205914ff7d0df9f753b393b0d5b9ad8f17) it was necessary to add some none generic condition into _drv_for_vif.
It would be better to solve it in some generic way.