Image Customizer: Set VHDX block-size to 2 MiB. (#10880)

Currently, the size of outputted VHDX files can be substantially larger than VHD or qcow2 files. This behavior occurs because `qemu-img` dynamically sets the block-size based on the size of the disk. And larger blocks sizes means the file format is less space efficient, which results in a larger image file.

This change sets the block-size to 2 MiB for VHDX files. This matches the `qemu-img` defaults for VHD and QCOW2.

For reference, Windows seems to use a default block-size of 1 MiB for VHDX files.

Author: cwize1

toolkit/tools/pkg/imagecustomizerlib/imagecustomizer.go

@@ -466,6 +466,12 @@ func toQemuImageFormat(imageFormat string) (string, string) {
 	case ImageFormatVhdFixed:
 		return QemuFormatVpc, "subformat=fixed,force_size"
 
+	case ImageFormatVhdx:
+		// For VHDX, qemu-img dynamically picks the block-size based on the size of the disk.
+		// However, this can result in a significantly larger file size than other formats.
+		// So, use a fixed block-size of 2 MiB to match the block-sizes used for qcow2 and VHD.
+		return ImageFormatVhdx, "block_size=2097152"
+
 	default:
 		return imageFormat, ""
 	}