vmdk: refuse enabling zeroed grain with flat images

This is a header flag and we needs sparse for the header.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/tests/qemu-iotests/059.out b/tests/qemu-iotests/059.out
index 265cd76..9b12efb 100644
--- a/tests/qemu-iotests/059.out
+++ b/tests/qemu-iotests/059.out
@@ -22,4 +22,8 @@
 file format: vmdk
 virtual size: 2.0G (2147483648 bytes)
 disk size: 4.0K
+
+=== Testing monolithicFlat with zeroed_grain ===
+qemu-img: TEST_DIR/t.IMGFMT: Flat image can't enable zeroed grain
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2147483648
 *** done