MAAS supports KVM in two different ways. The old way was via “Chassis” and the new way is “Fully managed KVM host (a.k.a pods)”.
In the chassis support, MAAS didn’t manage the host itself. It only discovered available machines, commissioned them, and made them available for deployment. However, on a “virsh” chassis, we allowed filtering via a prefix.
In the pod, the story is different. A pod is where MAAS fully manages the pod source. MAAS takes ownership of it as a whole, and all of the resources. If the user where to have pre-created VMs (or machines in the case of RSD), MAAS will take ownership of them given that it manages the infrastructure as a whole. In order to effectively have information about the machine to make it deployable, then MAAS will commission them. That said, IIRC, the API allows you to prevent commissioning all those pre-created VMs, but it is not surfaced on the UI because that’s not the desired behavior.
Furthermore, the support of KVM host as now supported relies on MAAS being able to create storage devices and network attachments, and for that MAAS needs to completely own the it. Hence the supported path to achieve that is by ensuring the MAAS has deployed the KVM host (which 2.5+ now supports).
Hope this helps.