bug#42315: `guix system vm-image` does not produce image with btrfs

[Jack Hill]

Hi Guix,

I'm using guix from commit: ec4c404c0575b340a04b3922fb828ac5417832dd

I want to create a vm-image with a btrfs filesystem. Following the manual 
for Invoking guix system 
<https://guix.gnu.org/manual/en/html_node/Invoking-guix-system.html>, I 
ran:

`guix system vm-image --image-size=120GB -t btrfs vm-config.scm`

with the operating system config below.

It produced a working vm image. However, the root file system was ext4.

Is this supposed to work, or did I misunderstand the documentation?

In an ideal world, I hope that this would work, but if we don't support it 
yet, I think that we should clarify the documentation, and make guix 
system vm-image error when passed a file system type it doesn't support.

Best,
Jack

(use-modules (gnu) (guix))
(use-service-modules networking ssh)
(use-package-modules bootloaders certs
                     package-management)


(operating-system
 (host-name "guix")
 (timezone "America/New_York")
 (locale "en_US.utf8")
 (initrd-modules (cons "virtio_scsi" %base-initrd-modules))

 ;; Label for the GRUB boot menu.
 (label (string-append "GNU Guix " (package-version guix)))

 (firmware '())

 ;; Below we assume /dev/vda is the VM's hard disk.
 ;; Adjust as needed.
 (bootloader (bootloader-configuration
              (bootloader grub-bootloader)
              (target "/dev/vda")
              (terminal-outputs '(console))))
 (file-systems (cons (file-system
                      (mount-point "/")
                      (device "/dev/vda1")
                      (type "btrfs")
                      (flags '(no-atime))
                      (options "compress=zstd"))
                     %base-file-systems))

 (users (cons (user-account
                (name "jackhill")
                (comment "Jack Hill")
                (group "users")
                (supplementary-groups '("wheel" "netdev"
               %base-user-accounts))

 ;; password-less sudo.
 (sudoers-file (plain-file "sudoers" "\
root ALL=(ALL) ALL
%wheel ALL=NOPASSWD: ALL\n"))

 (packages (append (list nss-certs)
                   %base-packages))

 (services
  (append (list (service openssh-service-type
                         (openssh-configuration
                          (password-authentication? #f)
                          (authorized-keys
                           `(("jackhill" ,(local-file
                                           
"/home/jackhill/.ssh/id_ed25519.pub"))))))

                ;; Use the DHCP client service rather than NetworkManager.
                (service dhcp-client-service-type))
          %base-services)))