Document how to run from a virtual USB storage in virt-manager from a USB image

Is there a plan to update the documentation of running Tails from a virtual USB storage in virt-manager (https://tails.boum.org/doc/advanced_topics/virtualization/virt-manager/index.en.html#index5h1)? The USB image can be used for this instead of using Tails Installer. The image must be increased to be at least 7200 MiB, I always do this with this command:

dd if=/dev/zero of=PATH_TO_TAILS.img conv=notrunc oflag=append bs=1000M count=6

FWIW one can get a simpler command line by using truncate.

Feature Branch: doc/16408-usb-image-virt-manager

Original created by @u on 16408 (Redmine)

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