core: ensure varlink Unit.List() can query inactive units

core: ensure varlink Unit.List() can query inactive units

manager_get_unit() only returns loaded units, if a unit is
stopped and not referenced by anything it will be GCed, so
this method will return nothing. Use manager_load_unit()
instead so that a stopped unit gets loaded and we can
return its state to the caller, after checking that it
actually exists.

Follow-up for d2a8dbabf5e2676f436515cc821040bd24eb6003