Cleanup VMs in template reverse topological order, not network one.
Network can be set to None to break dependency, but template can't. For
netvm to be changed, kill VMs first (kill doesn't check network
dependency), so netvm change will not trigger side effects (runtime
change, which could fail).
This fixes cleanup for tests creating custom templates - previously
order was undefined and if template was tried removed before its child
VMs, it fails. All the relevant files were removed later anyway, but it
lead to python objects leaks.
08ddeee9fb