bug#45974: guix system vm-image ignores filesystem type

[Jack Hill]

Hi Guix,

I believe that this is a feature request. I would like to create a Guix 
qcow2 image to run as a VM. Additionally, I would like the root file 
system to be btrfs. Therefore, I created an operating system definition 
with the following file-systems:

```
(file-systems (cons (file-system
                     (mount-point "/")
                     (device "/dev/vda1")
                     (type "btrfs")
                     (flags '(no-atime))
                     (options "compress=zstd"))
                    %base-file-systems))
```

I then ran `guix system vm-image --image-size=10G --volatile config.scm`. 
That successfully created a qcow2 image, but the root file system was ext4 
instead of btrfs.

Ideally, I would get an image with a btrfs filesystem. I expect supporting 
all combinations of filesystems and mapped devices might be challenging 
for the image creation process, but some more options might be 
interesting. At the very least, I think a warning or error when the 
requested configuration is not supported would be appropriate.

What do you think about this feature?

Best,
Jack