Disable data plane for scsi_cd

Signed-off-by: Nqingwangrh <qinwang@redhat.com>

qemu/tests/cfg/cdrom_block_size_check.cfg

@@ -5,6 +5,11 @@
     cdrom_without_file = yes
     excepted_qmp_err = "Device '${test_cdroms}' is locked and force was not specified, "
     excepted_qmp_err += "wait for tray to open and try again"
+    virtio_scsi:
+        # disable iothread
+        iothread_scheme ?=
+        iothreads ?=
+        image_iothread ?=
     Linux:
         check_cdrom_size_cmd = cat /sys/block/sr0/size
         mount_cdrom_cmd = "mount %s %s"