netdev-dummy: Include all dummy classes in iterations.

Commit 86f1d0326bd0 (netdev-dummy: Use netdev_get_devices() instead of a
local shash.) caused netdev-dummy functions that iterate over all dummy
devices to iterate only over the ones that have class 'dummy_class'.  This
seemed to obviously include all the ones that we want, but in fact
when ovs-vswitch is invoked with --enable-dummy=override, there are more
dummy classes than just dummy_class, which this new form of iteration
skipped over, with various negative consequences that showed up in some
testing.

This commit switches netdev-dummy back to internally tracking its own
dummy devices.  It fixes the tests for me.

Signed-off-by: Ben Pfaff <blp@nicira.com>