Did you create a new pool instead of the default one? The problem with the default settings of virt-manager is it stores .qcow2 images in /var/libvirt/images.
/var is the one part of the system that gets filled up quickly so you need to run
sudo apt clean
regularly.
In Devuan I created a separate pool and named it 'storage' in my /home folder. Had some issues in PCLOS using 'storage' so created a folder called Virtual Machines in my /home folder:
When it gets to selection time of a new .iso I don't choose the default location, I choose the Virtual Machines location and create a .qcow2 item:
I updated a post in Tutorials & Guides. Great video I added in this post: