software/kvm: fix when frontend-additional-instance-guid option doesn't exist

10 jobs for 1.0 in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:29:09

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:11:18

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP.own-distributor

100:40:39

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

13:28:39

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

04:44:25

passed SlapOS.SlapRunner.ResilienceTest-Master

00:35:18

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:09:42

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

11:52:02

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

05:57:15

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

03:22:49