event: Make debug message match function comments

The debug message said there was a timeout of 0 pending for -1 ms which
made me think this is where a hang was coming from but according to the
function comments this case means that there is no timeout pending so
make the debug message say that instead of saying there's a -1 ms
timeout.

src/util/vireventpoll.c

@@ -358,7 +358,10 @@ static int virEventPollCalculateTimeout(int *timeout) {
         *timeout = -1;
     }
 
+    if (*timeout > -1)
         EVENT_DEBUG("Timeout at %llu due in %d ms", then, *timeout);
+    else
+        EVENT_DEBUG("%s", "No timeout is pending");
 
     return 0;
 }