bridge: Tolerate missing Port and Interface records for local port.

Until now, ovs-vswitchd has been unable to configure IP addresses and
routes for bridges whose Bridge records lack a Port and an Interface
record for the bridge's local port (e.g. OFPP_LOCAL, the port with the
same name as the bridge itself).  When such a bridge was reconfigured,
ovs-vswitchd would output a log message that worried people.

This commit fixes the internal limitation that led to the message being
printed.

Bug #5385.

lib/ovsdb-idl.c

@@ -1108,6 +1108,15 @@ ovsdb_idl_get(const struct ovsdb_idl_row *row,
 
     return ovsdb_idl_read(row, column);
 }
+
+/* Returns false if 'row' was obtained from the IDL, true if it was initialized
+ * to all-zero-bits by some other entity.  If 'row' was set up some other way
+ * then the return value is indeterminate. */
+bool
+ovsdb_idl_row_is_synthetic(const struct ovsdb_idl_row *row)
+{
+    return row->table == NULL;
+}
 
 /* Transactions. */