Handle BH's queued by AIO completions in qemu_aio_flush()

Without this, the call to qemu_aio_flush during migration doesn't
actually flush all in-flight SCSI IOs.

Signed-off-by: Nolan Leake <nolan <at> sigbus.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

aio.c

@@ -112,7 +112,7 @@ void qemu_aio_flush(void)
         LIST_FOREACH(node, &aio_handlers, node) {
             ret |= node->io_flush(node->opaque);
         }
-    } while (ret > 0);
+    } while (qemu_bh_poll() || ret > 0);
 }
 
 void qemu_aio_wait(void)