feat: nested virtualization support per guest on the same kvm host

##### ISSUE TYPE

* Feature Idea

##### DESCRIPTION
As a CS user, I would like to be able to create a Guest / VM with `guest.cpu.mode=host-passtrough` and also a second VM without `guest.cpu.mode passtrough` on the same KVM host, that's not possible, because `guest.cpu.mode` is applied to the KVM host globally.

##### PROPOSAL
Custom compute offering `large.metal` ( similar to flavors in AWS with nested virtualization capability ) when one can set `allow.additional.vm.configuration.list.kvm` ( https://www.shapeblue.com/cloudstack-feature-first-look-enable-sending-of-arbitrary-configuration-data-to-vms/ ), probably doable with `` XML snippet ( see for more info https://libvirt.org/formatdomain.html#cpu-model-and-topology ) or with simple tick-checkbox for enabling nested virtualization.

It's important to have this ability directly on the offering to support for example CloudStack CAPI implementation, here https://github.com/kubernetes-sigs/cluster-api-provider-cloudstack/blob/main/templates/cluster-template.yaml#L72

Issue の custom compute offering proposal と参照されている cluster-template.yaml から始め、続いてリンク先の libvirt CPU model と topology のドキュメントを確認します。別の VM が同じ KVM host 上で host-passthrough モードなしに実行できる一方で、nested virtualization を offering または guest ごとに選択できれば完了です。

索引モデルが issue の本文から書いたものです。