bug#44468: 'guix system vm-image' crashes with 'string-prefix?: Wrong type argument in position 2 (expecting string)'

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

The full backtrace reads as:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix system  vm-image gnu/system/examples/desktop.tmpl --image-size=5G
Backtrace:
          19 (apply-smob/0 #<thunk 7fbaa93afc00>)
In ice-9/boot-9.scm:
    718:2 18 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8 17 (_ #(#(#<directory (guile-user) 7fbaa905cf00>)))
In guix/ui.scm:
  2116:12 16 (run-guix-command _ . _)
In ice-9/boot-9.scm:
  1736:10 15 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
  1731:15 14 (with-exception-handler #<procedure 7fba9d0fb420 at ice-9/boot-9.scm:1815:7 (exn)> _ #:unwind? _ #:unwind-for-type _)
In guix/scripts/system.scm:
   1342:8 13 (_)
In guix/status.scm:
    776:4 12 (call-with-status-report _ _)
In guix/scripts/system.scm:
   1175:4 11 (_)
In ice-9/boot-9.scm:
  1736:10 10 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In guix/store.scm:
   631:37  9 (thunk)
   1300:8  8 (call-with-build-handler #<procedure 7fba9aa6ac90 at guix/ui.scm:1167:2 (continue store things mode)> _)
  2042:24  7 (run-with-store #<store-connection 256.99 7fba9a493c30> _ #:guile-for-build _ #:system _ #:target _)
In guix/scripts/system.scm:
    814:2  6 (_ _)
    689:7  5 (_ #<store-connection 256.99 7fba9a493c30>)
In gnu/system/vm.scm:
    637:2  4 (system-qemu-image #<<operating-system> kernel: #<package linux-libre@5.9.3 gnu/packages/linux.scm:723 7fba9e649820> kernel-loadable-modules: () kernel-arguments: ("modprobe.blacklist=usbmouse,u…> …)
In unknown file:
           3 (remove #<procedure 7fba9ca0fa00 at gnu/system/vm.scm:640:12 (fs)> (#<<file-system> device: #<file-system-label "my-root"> mount-point: "/" type: "ext4" flags: () options: #f mount?: #t mount-…> …))
           2 (string-prefix? "/dev/" #<<uuid> type: fat bv: #vu8(205 171 52 18)> #<undefined> #<undefined> #<undefined> #<undefined>)
In ice-9/boot-9.scm:
  1669:16  1 (raise-exception _ #:continuable? _)
  1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
In procedure string-prefix?: Wrong type argument in position 2 (expecting string): #<<uuid> type: fat bv: #vu8(205 171 52 18)>
--8<---------------cut here---------------end--------------->8---

Thanks,

Maxim