software/kvm/test: store snapshots when custom tearDown fail

This shall help debugging test failures
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

03:00:22

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

04:10:17

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

05:30:07

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

06:20:36

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

00:00:10

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

04:00:23

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

03:59:25

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

00:40:43

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

05:12:06

passed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

03:56:19