It’s already possible to use ssh keys with virsh, as described in SSH setup fix.
The ssh-keys interface is not really desirable in this case, as it allows access to ssh keys for the user running the command/service in the snap (which would be root this case). This would allow MAAS to access all ssh keys configured for the root user, which might be a security issue.
By putting keys in /var/snap/maas/current/root/.ssh, only desider keys can be provided to MAAS.