Membership in the libvirt group is root-equivalent, not a permissions fix. Through qemu:///system it lets you attach arbitrary host block devices to a VM, so anyone in that group can mount and read the host's own disk. If you want to manage VMs without root or that group, use qemu:///session instead: unprivileged, images live under ~/.local/share/libvirt, no sudo. Tradeoff is you lose bridged networking without a setuid helper.