Make VM registration work unprivileged

Make VM registration work unprivileged

Since systemd 257, it's possible for unprivileged users to register
VMs with systemd-machined (gated behind polkit). So let's not require
root privileges to register virtual machines with systemd-machined, but
only do it as an unprivileged user if registration is explicitly requested
to avoid the polkit prompt.

To avoid having to detect which systemd version the system is running,
we skip errors on varlink registration for now when running unprivileged.
https://github.com/systemd/systemd/pull/36344 will allow us to detect if
the machined varlink interface can be accessed as an unprivileged user or
not.