varlink-io.systemd.MachineInstance,vmspawn: treat AddStorage/RemoveStorage name as...

varlink-io.systemd.MachineInstance,vmspawn: treat AddStorage/RemoveStorage name as opaque

The 'name' field on AddStorage and RemoveStorage was documented as
'<provider>:<volume>' and enforced via machine_storage_name_split() at
the varlink boundary. That form is only the convention machinectl
inherits from the StorageProvider routing path; the API itself only
needs a unique identifier the caller can re-use to detach the binding.

Drop the strict format check, require only a non-empty string, and
update the IDL docs to describe the field as a caller-supplied
identifier with machinectl's convention as a non-normative example.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>