ofproto-dpif: Query port existence by name to prevent warnings.

ofproto-dpif: Query port existence by name to prevent warnings.

The port_destruct() function checks if the port still exists in the
datapath to see if additional cleanup is necessary.  It used
dpif_port_query_by_number(), since the datapath port number is readily
available and cheap to use as a lookup handle.  Unfortunately, that
function logs a warning message if a port with that number doesn't exist
(because the datapath did remove it), which is confusing.  The
dpif_port_query_by_name() function doesn't log such an warning, so use
it instead.

Signed-off-by: Justin Pettit <jpettit@nicira.com>