block_iscsi_lvm: Disable iothread explicitly

Disable iothread explicitly for ppc64le and ppc64
since scsi-cd deivce doesn't support iothread.
Signed-off-by: NYongxue Hong <yhong@redhat.com>

qemu/tests/cfg/block_iscsi_lvm.cfg

@@ -56,6 +56,11 @@
     post_commands_iscsi += "${cmd_iscsi_delete_node},${cmd_iscsi_logout}"
     post_commands_lvm = "${cmd_remove_dm},${cmd_remove_pv},${cmd_remove_vg},${cmd_remove_lv}"
     post_command_noncritical = yes
+    ppc64le, ppc64:
+        # explicitly disable iothread
+        iothread_scheme ?=
+        image_iothread ?=
+        iothreads ?=
     variants:
         - with_scsi_hd:
             drive_format_image0 = scsi-hd