tools/image-info: deduplicate and sort the services

systemctl list-unit-files doesn't produce machine readable output.
parse_unit_files isn't very good at reading it and can produce duplicate
records.

This commit fixes it by deduplicating and sorting the units. This is a bit
hacky solution, but should work just alright. In the future we might
dump list-unit-files and do the job ourself, but let's not recreate the
systemd logic for now.

tools/image-info

@@ -67,6 +67,10 @@ def parse_unit_files(s, expected_state):
         if state != expected_state:
             continue
         r.append(unit)
+
+    # deduplicate and sort
+    r = list(set(r))
+    r.sort()
     return r