dpif-netdev: Make port numbers predictable for dummy dpif, for unit tests.

dpif-netdev: Make port numbers predictable for dummy dpif, for unit tests.

The unit tests feed a lot of flows through the ofproto-dpif "trace"
command, which means that they need to know the port numbers of the ports
that they create.  Until now, they've had to actually query those port
numbers from the database, which is a bit of unnecessary overhead for unit
tests.

This commit makes dummy dpif port numbers predictable: if the name of a
port contains a number, then the dummy dpif uses that number, if it is
valid and available, as the port number.

This commit also simplifies the unit tests that previously queried port
numbers to depend on the new behavior.

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