test suite: liveusb-creator hangs early

For me (anonym) liveusb-creator constantly hangs at "Unmounting /dev/sda" (at 0%, that is before the actual ISO content copy is done) when using the nested setup (not otherwise). If I take over the session with virt-viewer, close liveusb-creator and restart it, then it usually works. Also, doing something completely retarted like this also works much more often:

...
And I create a new 4 GiB USB drive named "A"
And I plug USB drive "A"
And I wait 10 seconds
And I unplug USB drive "A"
And I wait 10 seconds
And I plug USB drive "A"
And I wait 10 seconds
And I "Clone & Install" Tails to USB drive "A"
...

What can be the cause of this?

FTR, this still happens on 0.18~rc1.

Parent Task: #5978 (closed)

Related issues

• Related to #6092 (closed)

Original created by @tails on 6026 (Redmine)