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:
Thanks StarTreker,
I'm not sure how that applies (probably because I am new to all this). I've never played a computer game in my life. Other than work, my computer use is music/audio and photographic (some of both is professional). The purpose of the VM is solely for work which can't be done under Linux (yet).