kvm: Switch to dummy qcow2 images

The test does not require to download some enormous images, so provide small
empty qcow2 images, good enough to start the VM.
1 job for feature/kvm-drop-default-image in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-luke/fix/kvm-image-download-promise-fail

02:40:41