test-openflowd: Allow specifying port type on --ports option.

test-openflowd: Allow specifying port type on --ports option.

This allows a command like "test-openflowd --enable-dummy dummy@br0
--ports=dummy@eth0,dummy@eth1,dummy@eth2" to create a dummy datapath with
a number of dummy ports.  This is more useful for testing than a dummy
datapath with just an internal port, since output to "flood" and "normal"
has less pathological results.