qemu.tests: Add qemu_disk_img_commit test case

qemu/tests/cfg/qemu_disk_img.cfg

@@ -44,6 +44,20 @@
             image_chain += " sn1"
             image_name_sn1 = "images/sn1"
             image_format_sn1 = "qcow2"
+            variants:
+                - default:
+                - cache_mode:
+                    variants:
+                        - none:
+                            cache_mode = none
+                        - wirteback:
+                            cache_mode = writeback
+                        - unsafe:
+                            cache_mode = unsafe
+                        - directsync:
+                            cache_mode = directsync
+                        - writethrough:
+                            cache_mode = writethrough
         - rebase:
             type = qemu_disk_img_rebase
             rebase_mode = safe