All isotesters are broken due to guestfs freezing

Originally created by @anonym on #11896 (Redmine)

For all branches I see:

[...]
Feature: Untrusted partitions
  As a Tails user
  I don't want to touch other media than the one Tails runs from

  Scenario: Tails will not enable disk swap                       # features/untrusted_partitions.feature:6
    Given a computer                                              # features/step_definitions/common_steps.rb:122
    And I temporarily create a 100 MiB disk named "swap"          # features/step_definitions/common_steps.rb:139
23:14:21.566915970: libguestfs: trace: set_autosync true
23:14:21.567047844: libguestfs: trace: set_autosync = 0
23:14:21.567269186: libguestfs: trace: add_drive "/tmp/TailsToaster/TailsToasterStorage/swap" "format:qcow2"
23:14:21.567359204: libguestfs: trace: add_drive = 0
23:14:21.568114465: libguestfs: trace: launch
23:14:21.568203205: libguestfs: trace: get_tmpdir
23:14:21.568388347: libguestfs: trace: get_tmpdir = "/tmp/TailsToaster"
23:14:21.568504941: libguestfs: trace: get_backend_setting "force_tcg"
23:14:21.568615239: libguestfs: trace: get_backend_setting = NULL (error)
23:14:21.569047459: libguestfs: trace: get_cachedir
23:14:21.569195520: libguestfs: trace: get_cachedir = "/tmp/TailsToaster"
23:14:26.816791014: libguestfs: trace: get_backend_setting "gdb"
23:14:26.817021392: libguestfs: trace: get_backend_setting = NULL (error)

… and that’s when test suite run freezes and is aborted an hour later.

I actually saw this on a Debian Sid system while I was running the guestfs-based packages at version 1:1.32.7-1+b2, which was uploaded to jessie-backports just a few days ago, which would explain why it happened now. For me the issue seems to be fixed with 1.34.*, which unfortunately is not in jessie-packports or even stretch yet.