HCS: drop dependency on `qemu-img.exe` (used for converting qcow2/raw to vhdx)

https://github.com/lima-vm/lima/blob/dd909d0973cd84fa35f9e1693181b4585ea616c1/website/content/en/docs/config/vmtype/hcs.md?plain=1#L36-L49

HCS should not depend on external tools such as qemu-img.exe for converting a qcow2/raw image to VHDX.

---

• Option 1: implement a minimal VHDX writer in pure Go.

  • Spec: https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-vhdx/83e061f8-f6e2-4de1-91bd-5d518a43d477
  • Can be implemented in https://github.com/lima-vm/go-qcow2reader/blob/master/image/vhdx/vhdx.go

• Option 2: call CreateVirtualDisk, AttachVirtualDisk, and write to \\.\PhysicalDriveN

  • Needs admin (already needed anyway?)