Sort instances if possible inside an host aggregate

Simple use case: Reduce the licence cost
Windows licences are billed by host or by socket. The goal is to stack all Windows instances on the same hosts as much as possible without creating a new host aggregate.

As much as possible means this is a best effort feature, if nova can place this new Windows instance on a compute where other Windows are running, do it. If not, spawn it where you want or on a empty compute to identify it as « preferred widows compute » for future windows spawn.

This use case could be extended to many other situations, every time you have a preferred situation with no hard restrictions.