kvm: Update Debian versions

As Debian Bullseye has been released, set it as first option.

As Debian Buster 10.11 update has been released, upgrade it.
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:46:45

passed SlapOS.Eggs.UnitTest-Master.Python3

00:11:17

passed SlapOS.SlapRunner.ResilienceTest-Master

00:21:36