Allow hypervisor backend to choose a different image type from streams

[Saviq]

We currently always download the qcow2 image and convert it to the format the hypervisor supports.

Streams usually publish an image that can be used directly on most hypervisors, so it would be better to use those, especially since they may have better support for the respective hypervisor.

Ideally it would be a preference list, so that we can still use a different format if the preferred one is not available.

[Saviq]

Heh… this just got all the more important now that disk-disk.img is available in eoan streams.