dbus-sdr: Make VoltageRegulatorMode exposed to sensorTree.

getSensorSubtree() now search for
xyz.openbmc_project.Control.VoltageRegulatorMode
under the path /xyz/openbmc_project/vr.

Only the failure of searching sensor path is considered as an error.

Tested: ipmiStorageGetSDR()

This requires all the changes in
https://gerrit.openbmc-project.xyz/q/hashtag:%22dbus-sdr-vr-sensors%22+(status:open%20OR%20status:merged)

```
ipmitool sdr list event -v
Running Get PICMG Properties my_addr 0x20, transit 0, target 0
Error response 0xc1 from Get PICMG Properities
Running Get VSO Capabilities my_addr 0x20, transit 0, target 0
Invalid completion code received: Invalid command
Discovered IPMB address 0x0
Sensor ID              : vr_sensor_0_fprof (0xb4)
Entity ID              : 11.5 (Add-in Card)
Sensor Type            : Module / Board (0x15)

Sensor ID              : vr_sensor_1_fprof (0xb5)
Entity ID              : 11.6 (Add-in Card)
Sensor Type            : Module / Board (0x15)

...

```

Signed-off-by: Hao Jiang <jianghao@google.com>
Change-Id: I76d3fedb1cc0a2bc865c2a9c83c0418eb0ee99be
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/include/dbus-sdr/sdrutils.hpp b/include/dbus-sdr/sdrutils.hpp
index 8a3858c..753c0dd 100644
--- a/include/dbus-sdr/sdrutils.hpp
+++ b/include/dbus-sdr/sdrutils.hpp
@@ -213,6 +213,17 @@
 // sensors. These objects are global singletons, used from a variety of places.
 inline IPMIStatsTable sdrStatsTable;
 
+/**
+ * Search ObjectMapper for sensors and update them to subtree.
+ *
+ * The function will search for sensors under either
+ * /xyz/openbmc_project/sensors or /xyz/openbmc_project/extsensors. It will
+ * optionally search VR typed sensors under /xyz/openbmc_project/vr
+ *
+ * @return the updated amount of times any of "sensors" or "extsensors" sensor
+ * paths updated successfully, previous amount if all failed. The "vr"
+ * sensor path is optional, and does not participate in the return value.
+ */
 uint16_t getSensorSubtree(std::shared_ptr<SensorSubTree>& subtree);
 
 bool getSensorNumMap(std::shared_ptr<SensorNumMap>& sensorNumMap);