datapath-windows: Update OVSEXT VS project to support 6.40

Added support for creating OVS extension driver NDIS 6.40 compliant.

Currently the OVSEXT Visual Studio project has four build configurations,
'Win8 Release', 'Win8 Debug', 'Win8.1 Release' and 'Win8.1 Debug'. All of
them are creating a binary that is NDIS 6.30 compliant. I have changed the
Win8.1 build configurations in order to create a binary that is NDIS 6.40
compliant.

In this way, the OVSEXT project is able to create a release/debug binary
that is NDIS 6.30 compliant, using the 'Win8 Release' and 'Win8 Debug'
build configurations, as well as a release/debug binary that is NDIS 6.40
compliant, using the 'Win8.1 Release' and 'Win8.1 Debug' build
configurations.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>