scsi: use scsi_req_cancel_async when purging requests

This avoids calls to aio_poll without having acquired the context first.
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <1450290827-30508-1-git-send-email-pbonzini@redhat.com>

hw/scsi/scsi-bus.c

@@ -1841,11 +1841,13 @@ void scsi_device_purge_requests(SCSIDevice *sdev, SCSISense sense)
 {
     SCSIRequest *req;
 
+    aio_context_acquire(blk_get_aio_context(sdev->conf.blk));
     while (!QTAILQ_EMPTY(&sdev->requests)) {
         req = QTAILQ_FIRST(&sdev->requests);
-        scsi_req_cancel(req);
+        scsi_req_cancel_async(req, NULL);
     }
-
+    blk_drain(sdev->conf.blk);
+    aio_context_release(blk_get_aio_context(sdev->conf.blk));
     scsi_device_set_ua(sdev, sense);
 }