guest-ssh-* command support

[jbpratt]

Hi! Thank you for your effort on the port.

I'm working on getting FreeBSD running on (read managed by) Kubernetes using KubeVirt. The guest-ssh-{get,add,remove}-authorized-keys commands allow for dynamic injection (at scale) of SSH keys. This feature was added in 5.2 according to the qapi-schema. Current upstream version is 6.1.93 which based on a comment in a previous thread, is probably too large of a task to tackle at the moment.