kvm: Improve boot image url handling

image-download-controller raises error state during download process, to
inform other components that download is not ready yet.

template-kvm-run.in asserts presence of all images and otherwise refuses to
start, so that it gives chance for the image to being downloaded.

Sorting images happen by proper key instead of needless links.

Fix race condition, as the hash parameter shall depend on boot image url
configuration, and not it's failure state.