feat: add disk_format config with post-export disk conversion

[abdelhaleemahmed]

Summary

• Adds a disk_format configuration field to the virtualbox-iso builder
• Supports VDI, VMDK, and VHD output formats
• After VBoxManage export (which always produces VMDK), automatically converts to the requested format using VBoxManage clonemedium
• Updates OVF references to point to the new disk file

Problem

VBoxManage export always converts VDI disks to VMDK — this is hardcoded in VirtualBox's C++ source (ApplianceImplExport.cpp). Users who want VDI output (for discard/compact support) have no way to get it from the builder.

Solution

Rather than trying to bypass VBoxManage export, this PR adds a post-export conversion step:

1. VBoxManage export runs normally → produces correct OVF + VMDK
2. If disk_format is not VMDK, convertExportedDisk() runs:
   • VBoxManage clonemedium disk.vmdk disk.vdi --format VDI
   • Updates .vmdk references in the OVF to .vdi
   • Deletes the VMDK

Usage

source "virtualbox-iso" "example" {
  disk_format = "VDI"
  # ... other config
}

Default is VDI (unchanged from current behavior where VirtualBox creates VDI internally). When set to VMDK, no conversion occurs (existing behavior).

Changes

• builder/virtualbox/common/step_export.go — Added DiskFormat field to StepExport, added convertExportedDisk() method
• builder/virtualbox/iso/builder.go — Added DiskFormat config field with validation, pass to StepExport
• builder/virtualbox/iso/step_create_disk.go — Use config.DiskFormat instead of hardcoded "VDI"
• builder/virtualbox/iso/builder.hcl2spec.go — Added DiskFormat to HCL2 spec

Test plan

• Build Rocky Linux 9 box with disk_format = "VDI" — VDI in output
• Verify OVF references updated correctly
• Vagrant post-processor packages VDI into .box
• vagrant up imports and boots successfully
• Build with default (no disk_format) — existing behavior unchanged
• Build with disk_format = "VMDK" — no conversion occurs

🤖 Generated with Claude Code

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