If ubuntu also uses selinux you can try restorevon -v /var/lib/libvirt/images/disk.qcow2.

If I create a disk through libvirt storage management on Fedora the permissions are:
$ sudo ls -l /var/lib/libvirt/images/test.qcow2
-rw-------. 1 root root 21478375424 Jul 28 00:42 /var/lib/libvirt/images/test.qcow2

If you created it by hand perhaps adjust the permissions or delete and recreate it through virt-manager.

I'll caution you'll want to use at least qemu 8 to have working audio and network. I believe both are a definite no go prior. 8.1 fixes an issue shutting down.