info reports wrong disk numbers for core-based instances

[ricab]

$ multipass info zealous-cusk firm-harrier | egrep -i "name|release|disk"
Name:           firm-harrier
Release:        Core 18
Disk usage:     54,6M out of 54,6M
Name:           zealous-cusk
Release:        Core 16
Disk usage:     89,1M out of 89,1M

These values are being obtained from df for /:

ubuntu@firm-harrier:~$ df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0       55M   55M     0 100% /

However, things are actually in /writable:

ricab@ricab-xps13:~ $ multipass exec firm-harrier -- df -h /writable
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda3       4.9G  560M  4.1G  12% /writable
ricab@ricab-xps13:~ $ multipass exec firm-harrier -- fallocate -l 1G test
ricab@ricab-xps13:~ $ multipass exec firm-harrier -- df -h /writable
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda3       4.9G  1.6G  3.1G  34% /writable