Revert "kvm: Bang immediately in case of vnc_promise problem"

This reverts commit d7721eb4.

The change didn't fix the real problem.
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:10:09

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:00

passed SlapOS.SlapRunner.ResilienceTest-Master

00:21:37

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:26:46

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

04:22:45

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

13:48:36