instack-virt-setup assumes images path is the default

This command caused trouble for me:

```
tripleo configure-vm --name instack --image /var/lib/libvirt/images/instack.qcow2 --seed --libvirt-nic-driver virtio --arch x86_64 --memory 2097152 --cpus 1
```

My images location differs from the default one, so the vm won't boot
