As a libvirt API user, I want to be able to request a CPU configuration update for a VM, so that the VM configuration is updated with security fixes if they are compatible with one or more hosts.

Requirements:

• A CPU update shouldn't require management software updates. Ideally, a kernel and QEMU update should be enough to make a CPU model update available through the API.
• The input for the API may contain the capabilities for a set of hosts (not just the current hosts). If the VM configuration is runnable on those hosts, the VM configuration update must stay runnable.
• The mechanism shouldn't require the machine-type for the VM to be changed.

Design and planning document: https://docs.google.com/document/d/1yWcWecZgGdHLL6j3Otd34sm3rZNln0SzkPvSIHt0yQM/edit#