proper destruction of netdev datapaths

proper destruction of netdev datapaths

Bridges with datapath_type=netdev do not destroy the datapath when
deleted. In particular, the tap device implementing the internal
interface is not close()d, and therefore the tap persists until
ovs-vswitchd exit()s.

This behaviour appears to be caused by the missing callback
for 'enumerate' in the dpif-netdev class. Without this callback
'bridge_reconfigure' fails to realize that there are datapaths with no
bridge, and thus cannot destroy them. Providing an 'enumerate' callback
fixes this.