Hyperkit: use raw disk format instead of qcow2

[guillaumerose]

Few years ago, Apple started to ship macOS with APFS. It supports sparse files allowing us to use raw file format.

According to Docker Desktop release notes, we can expect a factor 2 for disk performance:

For systems running APFS on SSD on High Sierra, use raw format VM disks by default. This improves disk throughput (from 320MiB/sec to 600MiB/sec in dd on a 2015 MacBook Pro) and disk space handling.

---

Needed changes:

• Allow the use of raw images for Hyperkit snc#211
• Modify imageformat according to file used machine-driver-hyperkit#11

[cfergeau]

There was a short attempt in snc which was then reverted crc-org/snc@34ca27c
Apart from minor snc/machine-driver-hyperkit bugs, there were issues with crc not dealing correctly with sparseness, in particular during the VM image copy we do on start, which made startup much longer than before, in addition to losing sparseness benefits.

[gbraad]

The PR and revert is recorded as: crc-org/snc#206 an crc-org/snc#208. I mentioned them in the previous snc related issue. It was unfortunate to see an optmistic merge which then gets reverted without clearly recording the cause and what is needed in future to solve this. This is why I added this notes and issues.