AIO deletion race fix

When deleting an fd event there is a chance the object doesn't get
deleted, but only ->deleted set positive and deleted somewhere later.

Now, if we create a handler for the fd again before the actual
deletion occurs, we end up writing data into an object that has
->deleted set, which is obviously wrong.

I see two ways to fix this:

1. Don't return ->deleted objects in the search
2. Unset ->deleted in the search

This patch implements 1. which feels safer to do. It fixes AIO issues
I've seen with curl, as libcurl unsets fd event listeners pretty
frequently.
Signed-off-by: NAlexander Graf <alex@csgraf.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

aio.c

@@ -44,7 +44,8 @@ static AioHandler *find_aio_handler(int fd)
 
     LIST_FOREACH(node, &aio_handlers, node) {
         if (node->fd == fd)
-            return node;
+            if (!node->deleted)
+                return node;
     }
 
     return NULL;