Merge pull request #1311 from pingl2016/1565424

qemu_disk_img.commmit: Update configurations for the case

qemu/tests/cfg/qemu_disk_img.cfg

@@ -42,6 +42,8 @@
                 - writeback:
                     cache_mode = writeback
                 - unsafe:
+                    Host_RHEL.m6:
+                        no commit
                     cache_mode = unsafe
                 - directsync:
                     cache_mode = directsync
@@ -122,6 +124,8 @@
             image_chain += " sn1"
             image_name_sn1 = "images/sn1"
             image_format_sn1 = "qcow2"
+            backup_image_before_testing = yes
+            restore_image_after_testing = yes
         - info:
             type = qemu_disk_img_info
             guest_file_name_image1 = "${tmp_dir}/test.img"