Tails Upgrader mistakenly identifies read-write USB boot device as a DVD or a read-only device

Originally created by @intrigeri on #12501 (Redmine)

I’ve started Tails 3.0~beta3 from a (virtual, removable) USB stick in libvirt/KVM, and it tells me:

You should do a manual upgrade to Tails 3.0~beta4.

For more information about this new version, go to https://tails.boum.org/news/test_3.0-beta4/

It is not possible to automatically upgrade your device to this new version: Tails was started from a DVD or a read-only device.

Someone else reported the same issue on tails-testers@. It’s weird that other Tails contributors didn’t report it yet, so perhaps this doesn’t happen all the time?

Feature Branch: bugfix/12501-second-automatic-upgrade