ofproto: Make OpenFlow connection log messages name the datapath.

ofproto: Make OpenFlow connection log messages name the datapath.

Until now, log messages about OpenFlow connections have named the target
of the connection, e.g. "tcp:1.2.3.4:5555", but they have not named the
datapath.  Most often, every datapath has the same target, so this can
make it difficult to tell which connection is going wrong.  Usually, that
isn't important, because all connections with the same target will have the
same problems, but it's probably better to be more informative.

This commit changes the log messages to include the datapath name, so that
"tcp:1.2.3.4:5555" becomes, e.g., "xenbr0<->tcp:1.2.3.4:5555".

Requested-by: Keith Amidon <keith@nicira.com>