core/device: add stopping job message

The use case for stopping a device unit is indeed narrow,
but we still want to show a clear message.

Preparation for later commits.

(cherry picked from commit 3f4a7a472f90949425f76ffde39a6c94a0b77e0b)
(cherry picked from commit 7feb10fdda886d50fa7ba2e252b31ba815411893)
(cherry picked from commit cd24f42fe12673eb9001ad74e842bdd2ba80a206)

src/core/device.c

@@ -1285,6 +1285,7 @@ const UnitVTable device_vtable = {
         .status_message_formats = {
                 .starting_stopping = {
                         [0] = "Expecting device %s...",
+                        [1] = "Waiting for device %s to disappear...",
                 },
                 .finished_start_job = {
                         [JOB_DONE]       = "Found device %s.",