Virtualbox shared folders are not automatically mounted in Tails 3.0

Originally created by @goupille on #12724 (Redmine)

a user reported that since upgrading to tails 3.0, it wasn’t possible to mount shared folders with virtualbox. the issue didn’t occur in 3.0~rc1.
this user was affected by #12357 (closed) since a few releases, but shared folders worked at that time, so it doesn’t seems related.