It would be really great if you could build a library of qcow2 templates (in the same way you can tar.gz ones for CTs) which you then assign to VMs as a template and it creates the disk using -o backing_file=<template>. This would mean you can have an army of Windows boxes with little extra overhead on disk. The system could also keep track of which VMs you have that use a particular backing file, also a way to promote a new template by copying the template to a new name, unsafe rebasing the shutdown VM and then committing it.