Improve pf layer

+ Merge single/double link
+ Introduce _pf{n}[0] variables so we can access physical function
  same way as virtual function
+ Cleanup code by moving complex logic to python
+ Prepare code for multiple vf functions

Signed-off-by: Peter Mikus <pmikus@cisco.com>
Change-Id: Ic2e74a38bfa146441357de8f0916aeb638941c49