Add qcow2 support

[NateChoe1]

This PR adds qcow2 disk image support while maintaining backwards compatibility. qemu-img does have support for raw image files, but I chose not to use that so the ALLOCATE flag would keep working. A lot of code could be removed by converting everything to use qemu-img, though.

[kroese]

Thank you very much!! But two days ago I did a force push to the main branch, causing all existing forks to become invalid. I did not expect anyone was working on something, but I see your fork is from before that date.

Is it possible that you create a fresh fork and copy the three files you modified to that one? Because I cannot merge this one without breaking the master.

Thanks in advance!

[kroese]

Also when switching the DISK_FMT flag on an existing install, it will not cleanup the old data.img but just creates a new data.qcow.

Would be really that when a data file is detected with the opposite extension to DISK_FMT, it would run qemu-img convert to convert from raw to qcow (and vice versa) instead of abandoning the existing disk and creating a new one. So that you can switch the flag without loosing any data.

I will see if I have the time to add that function after the pullrequest gets merged.

[NateChoe1]

Moved to #226