Add support for Gen 2 Hyper-V VMs.
`./image_to_vm` tool has now a new supported format: `hyperv_vhdx`,
that produces .vhdx dynamic disks.
How to use:
```bash
./image_to_vm.sh --from ../build/images/amd64-usr/developer-latest/ --format hyperv_vhdx
```
See: https://github.com/flatcar/Flatcar/issues/1009
Uses PR: https://github.com/flatcar/bootengine/pull/92
Signed-off-by: Adrian Vladu <avladu@cloudbasesolutions.com>