Allow hypervisor backend to choose a different image type from streams

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.

Trace the current qcow2 download and conversion path, then inspect how the hypervisor backend selects an image from streams. Define a preference order for supported image types with fallback when a preferred format is unavailable. Done means the selected image can be used directly when available and conversion still works as a fallback.

Written by the indexing model from the issue text.