Fixes:

(gnome-boxes:9304): Boxes-WARNING **: 19:54:29.345: review-page.vala:32: Box setup failed: Failed to create volume: internal error: creation of non-raw file images is not supported without qemu-img.

(gnome-boxes:9304): Boxes-CRITICAL **: 19:54:29.345: boxes_assistant_review_page_populate: assertion 'machine != NULL' failed

Steps to reproduce:
1. Open gnome-boxes on GNU Guix
2. Try to create a box with GNU Guix ISO and expect failure above