xentrace: Fix race in updating consumer ring index.

From: George Dunlap <gdunlap@xensource.com>
Signed-off-by: Keir Fraser <keir@xensource.com>

tools/xentrace/xentrace.c

@@ -394,7 +394,7 @@ int monitor_tbufs(int outfd)
             }
 
             mb(); /* read buffer, then update cons. */
-            meta[i]->cons = meta[i]->prod;
+            meta[i]->cons = prod;
         }
 
         nanosleep(&opts.poll_sleep, NULL);