aio-posix: Improve comment around marking node deleted

The counter is for qemu_lockcnt_inc/dec sections (read side),
qemu_lockcnt_lock/unlock is for the write side.
Suggested-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <20180803063917.30292-1-famz@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>

util/aio-posix.c

@@ -232,7 +232,7 @@ void aio_set_fd_handler(AioContext *ctx,
             g_source_remove_poll(&ctx->source, &node->pfd);
         }
 
-        /* If the lock is held, just mark the node as deleted */
+        /* If a read is in progress, just mark the node as deleted */
         if (qemu_lockcnt_count(&ctx->list_lock)) {
             node->deleted = 1;
             node->pfd.revents = 0;