bug#35472: Boot bind mount device path incorrect

[rendaw <7e9wc56emjakcm@s.rendaw.me>]

Package: guix
Version: 0.16.0

I specified a needed-at-boot bind mount from "/x/y" to "/y" but this
failed with the error:

ERROR: In procedure stat:
In procedure stat: No such file or directory: "/x/y"

At boot "/x" is actually mounted at "/root/x" so the bind mount path
should be "/root/x/y" -- making such a change causes the boot to work.

User defined filesystems with non-dev paths added to the boot mounts
should either automatically get "/root" prepended to the device path or
else the fact that they need to be prepended should be included in the
documentation, since there's no other way to discover this (other than
reading the source and trial/error).

For people hitting the same error, you can double check the mount points
at the repl you're dropped into with:

(use-modules (ice-9 ftw))
(scandir "/root")