core: add IODeviceLatencyTargetSec

This adds support for the following proposed latency based IO control
mechanism.

  https://lkml.org/lkml/2018/6/5/428

src/systemctl/systemctl.c

@@ -4899,6 +4899,28 @@ static int print_property(const char *name, sd_bus_message *m, bool value, bool
 
                         return 1;
 
+                }  else if (contents[0] == SD_BUS_TYPE_STRUCT_BEGIN &&
+                            streq(name, "IODeviceLatencyTargetUSec")) {
+                        char ts[FORMAT_TIMESPAN_MAX];
+                        const char *path;
+                        uint64_t target;
+
+                        r = sd_bus_message_enter_container(m, SD_BUS_TYPE_ARRAY, "(st)");
+                        if (r < 0)
+                                return bus_log_parse_error(r);
+
+                        while ((r = sd_bus_message_read(m, "(st)", &path, &target)) > 0)
+                                print_prop(name, "%s %s", strna(path),
+                                           format_timespan(ts, sizeof(ts), target, 1));
+                        if (r < 0)
+                                return bus_log_parse_error(r);
+
+                        r = sd_bus_message_exit_container(m);
+                        if (r < 0)
+                                return bus_log_parse_error(r);
+
+                        return 1;
+
                 } else if (contents[0] == SD_BUS_TYPE_BYTE && streq(name, "StandardInputData")) {
                         _cleanup_free_ char *h = NULL;
                         const void *p;