sysupdated: don't crash when an mstack machine image is found

As soon as machinectl list-images has an mstack entry updatectl fails
because systemd-sysupdated crashes with an assertion failing because
the mstack case was not handled.
For now mstack is not supported as image for sysupdate to operate on
and we can skip it.

Fixes https://github.com/systemd/systemd/issues/41649

src/sysupdate/sysupdated.c

@@ -1882,6 +1882,9 @@ static int manager_enumerate_image_class(Manager *m, TargetClass class) {
                 if (image_is_host(image))
                         continue; /* We already enroll the host ourselves */
 
+                if (image->type == IMAGE_MSTACK)
+                        continue; /* systemd-sysupdate doesn't support mstack images yet */
+
                 r = target_new(m, class, image->name, image->path, &t);
                 if (r < 0)
                         return r;