Update doc wrt Tails in virt-manager in Stretch

There’s one small change in the section “Running Tails from an ISO image”; Version: Jessie is now Version: Stretch. The rest remained unchanged.

Affects doc/advanced_topics/virtualization/virt-manager.mdwn.

The file doc/advanced_topics/virtualization/media-playback-start.png could be moved to lib/ while doing this, but there’s already a file called media-playback-start.png in there. The difference between both is that the one in /lib is black and the one used in virt-manager.mdwn is grey. Maybe it should be renamed and moved to /lib anyway?

Feature Branch: spriver:doc/12380-virt-manager-in-stretch

Parent Task: #11916 (closed)

Original created by @spriver on 12380 (Redmine)