datapath-windows: Updated WFP system provider handling

datapath-windows: Updated WFP system provider handling

If the Base Filtering Engine (BFE) is not started, the WFP system
provider failed to be added because no session to the engine could
be acquired.

The solution for this was to registered a BFE notification callback
that is called whenever the BFE's state changes. Only if the BFE's
state is running the WFP system provider is added.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/65
Acked-by: Eitan Eliahu  <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>