Jenkins' isotesters fail to create VMs

Most tests (since we need a VM for almost all tests) are failing like this:

  Scenario: Tails will not enable disk swap                       # features/untrusted_partitions.feature:6
    Given a computer                                              # features/step_definitions/common_steps.rb:46
    And I temporarily create a 100 MiB disk named "swap"          # features/step_definitions/common_steps.rb:59
    And I create a gpt swap partition on disk "swap"              # features/step_definitions/untrusted_partitions.rb:1
    And I plug sata drive "swap"                                  # features/step_definitions/common_steps.rb:66
    When I start Tails with network unplugged and I login         # features/step_definitions/common_steps.rb:118
      Call to virDomainCreateWithFlags failed: internal error: process exited while connecting to monitor: 2019-06-26T07:52:47.558939Z qemu-system-x86_64: Property '.md-clear' not found (Guestfs::Error)
      ./features/support/helpers/vm_helper.rb:693:in `create'
      ./features/support/helpers/vm_helper.rb:693:in `/^I start the computer$/'

Original created by @anonym on 16843 (Redmine)