software/kvm: Drop default image and replace it with boot-image-url-select

boot-image-url-select is used instead of default image being downloaded by the
software release.

If nothing is selected, the default boot-image-url-select is used, but not if
other way to obatin boot image is enabled.
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

04:04:48

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

04:32:51

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

03:43:20

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

04:05:15

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

04:04:16

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

04:21:53

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

03:41:34

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

03:59:57