datapaths: Clean up some string formatting

This switches to using log's string substitution as well
as cleaning some other things up.
datapaths: Insert some newlines

Slightly better conformance to POX coding style.
tests: Remove a print statement
tests: Update class name of software datapath
openflow.software_switch: changing the way port_down() is implemented

we used to delete the port_no. This is somewhat awkward if the client is
trying to get a ofp_phy_port object from the switch.

We know maintain a set `down_port_nos` rather than modifying self.ports
software switch and friends: refactored handler structure

now OFConnection invokes a single callback function
  on_message_received(connection, message)
when an OpenFlow message has been received and decoded. The
multiplexing to different handlers for different types hapens
in the switch.
Making a setter for on_message_received
datapaths: Fix port_mod/ofp rename

_receive_port_mod() seems to have changed it parameter name from
'ofp' to the more specific 'port_mod', but it looks like this
patch wasn't completely applied.  Fixed.
datapaths: Fix imports on nx_switch

This updates it to the new package name.
tests: Update MockSwitch (private xid_generator)

I think the API should actually have a new_xid() method or
something, but this fixes things for now.