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.
11 jobs for master in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:07:12

passed SlapOS.Eggs.UnitTest-Master.Python2

00:11:56

passed SlapOS.Eggs.UnitTest-Master.Python3

00:15:34

passed SlapOS.SlapRunner.ResilienceTest-Master

00:22:47

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:30:21

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

04:54:45

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

04:53:15

passed SlapOS.Eggs.UnitTest-Master.Python2

00:11:58

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:45

passed SlapOS.SlapRunner.ResilienceTest-Master

00:22:32

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:29:18