get rid of a guest,
virsh undefine virtinstall rm -f /data/kvm/virtinstall.qcow2
to rename a guest,
virsh domrename ... ...
but you might also want to rename the vdisk and change the pointer to it in the same run,
virsh shutdown guest virsh dumpxml guest > lala virsh undefine guest vi lala virsh define lala rm lala