dpif-netdev: allow for proper destruction of netdev datapaths

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

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

Signed-off-by: Giuseppe Lettieri <g.lettieri@iet.unipi.it>
Signed-off-by: Ben Pfaff <blp@nicira.com>