datapath-windows: Rename files.

This patch includes the file renaming and accommodations needed for the file
renaming to build the forwarding extension for Hyper-V.

This patch is also a follow-up for the thread:
http://openvswitch.org/pipermail/dev/2014-August/044005.html

Signed-off-by: Samuel Ghinet <sghinet@cloudbasesolutions.com>
Co-authored-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>