datapath-windows: Support for hybrid forwarding

In a hybrid forwarding environment, there are two types of packets that
enter and leave the Hyper-V extensible switch: NVGRE packets and non-NVGRE
packets. Hybrid forwarding involves filtering the incoming traffic based
on packet type.

Thus, we must split the incoming traffic into NVGRE and non-NVGRE packets.
All non-NVGRE traffic is forwarded by the OVS extension and processed as
usual, and the NVGRE traffic is passed to NDIS to be handled by the HNV
module.

Necessary VS project changes regarding the compiler settings will be made
in a different patch.

More details about hybrid forwarding and the necessary NDIS 6.40 support
is provided in issue #52.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Nithin Raju <nithin@vmware.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/52
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>