Merge pull request #2110 from xutian/inc_backup2

add cases to test backup image with different cluster_size

qemu/tests/cfg/blockdev_inc_backup_cluster_test.cfg

+- blockdev_inc_backup_cluster:
+    type = blockdev_inc_backup_test
+    virt_test_type = qemu
+    images += " data"
+    image_chain_data = "base inc"
+    force_remove_image_image1 = no
+    force_create_image_image1 = no
+    force_create_image_data = yes
+    force_remove_image_data = yes
+    start_vm = no
+    storage_pools = default
+    storage_pool = default
+    storage_type_default = "directory"
+
+    image_size_data = 2G
+    image_size_base = 2G
+    image_size_inc = 2G
+
+    image_format_data = qcow2
+    image_format_base = qcow2
+    image_format_inc = qcow2
+
+    image_name_data = data
+    image_name_base = base
+    image_name_inc = inc
+    source_images = "data"
+    rebase_mode = unsafe
+    only Linux
+    qemu_force_use_drive_expression = no
+    variants:
+        - inc_cluster_size:
+            variants:
+                - size_512:
+                    image_cluster_size_inc = 512
+                - size_4K:
+                    image_cluster_size_inc = 4096
+                - size_2M:
+                    image_cluster_size_inc = 2097152
+                - @size_default: