restruct the backend storage command submit

pkg/scsi/backingstore/null.go

@@ -59,7 +59,18 @@ func (bs *NullBackingStore) Size(dev *api.SCSILu) uint64 {
 	return 0
 }
 
-func (bs *NullBackingStore) CommandSubmit(cmd *api.SCSICommand) error {
-	cmd.Result = api.SAM_STAT_GOOD
+func (bs *NullBackingStore) Read(offset, tl int64) ([]byte, error) {
+	return nil, nil
+}
+
+func (bs *NullBackingStore) Write(wbuf []byte, offset int64) error {
+	return nil
+}
+
+func (bs *NullBackingStore) DataSync() error {
+	return nil
+}
+
+func (bs *NullBackingStore) DataAdvise(offset, length int64, advise uint32) error {
 	return nil
 }