aio: fix aio_ctx_prepare with idle bottom halves

Commit ed2aec4867f0d5f5de496bb765347b5d0cfe113d changed the return
value of aio_ctx_prepare from false to true when only idle bottom
halves are available.  This broke PC old-style DMA, which uses them.
Fix this by making aio_ctx_prepare return true only when non-idle
bottom halves are scheduled to run.
Reported-by: Nmalc <av1474@comtv.ru>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Nmalc <av1474@comtv.ru>

async.c

@@ -122,11 +122,9 @@ aio_ctx_prepare(GSource *source, gint    *timeout)
 {
     AioContext *ctx = (AioContext *) source;
     QEMUBH *bh;
-    bool scheduled = false;
 
     for (bh = ctx->first_bh; bh; bh = bh->next) {
         if (!bh->deleted && bh->scheduled) {
-            scheduled = true;
             if (bh->idle) {
                 /* idle bottom halves will be polled at least
                  * every 10ms */
@@ -135,12 +133,12 @@ aio_ctx_prepare(GSource *source, gint    *timeout)
                 /* non-idle bottom halves will be executed
                  * immediately */
                 *timeout = 0;
-                break;
+                return true;
             }
         }
     }
 
-    return scheduled;
+    return false;
 }
 
 static gboolean