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

refs.