dpif-netdev: enumerate dpif belonging to the right class

Since dpif_netdev_enumerate() is used for "netdev" and "dummy" class, it
incorrectly lists dpif-netdevs as "dummy" and vice versa.
This patches address the issue by changing the dpif-provider interface: a
dpif_class parameter is passed to the 'enumerate' call to match the right class.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>