The first important stuff would be to have in place the metrics of consumption of the
resources some hoster "sell" to a customer. So usage metrics on
CPU / RAM /  DISK / Network (over provisioned or not is another story)
There is an aspect on actual usage versus provisioned usage.
{vm provisioned 10 GB of RAM bus os+app uses just 5 GB of them etc)
But let's keep it simple.

Should we have the usage reference then a GUI that the can define
the cost plans, would be ideal. A way to query if possible that
database, would ease existing ERPs and invoicing software to
produce / send invoices. This is for mostly post paid use cases.

For prepaid situations, there must be some regular accounting that
will stop the service through our current API. It sounds a bit scaring to
actually shutdown the provisioned vm. So at least on first step stop the
networking ....
I guess that the base is to create a separate database that will keep the
consumed metrics, so that customers could actually use it to
enforce some business logic. (as explained above).
Should there be a minimal gui that could a small hoster create
his owns plans and perhaps issue some "invoice" would be ideal.
Since there are a number of open source billing systems out there,
perhaps the integration is not that complicated. A second thought would
be to extend the data ware house gui to create the "invoice" outputs
as most of the usage info is there.
I know that ovirt is supposed to be a virtualization solution and not
a full monty billing system.