ups: watch for property changes

Monitor for any changes to the properties the chassis manger is
interested in with the UPower interface.

Tested:
- Changed the properties via busctl after the chassis manager was
  running and verified the chassis power status was updated correctly

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I13f23d33581c2339506f9c4739ce36bf07aef46e
diff --git a/chassis_state_manager.hpp b/chassis_state_manager.hpp
index 9be2b39..3b58fcd 100644
--- a/chassis_state_manager.hpp
+++ b/chassis_state_manager.hpp
@@ -135,6 +135,9 @@
     /** @brief Used to subscribe to dbus systemd signals **/
     sdbusplus::bus::match_t systemdSignals;
 
+    /** @brief Watch for any changes to UPS properties  **/
+    std::unique_ptr<sdbusplus::bus::match_t> uPowerPropChangeSignal;
+
     /** @brief Used to Set value of POHCounter */
     uint32_t pohCounter(uint32_t value) override;
 
@@ -204,6 +207,16 @@
      *  @return true if fault detected, else false
      */
     bool standbyVoltageRegulatorFault();
+
+    /** @brief Process UPS property changes
+     *
+     * Instance specific interface to monitor for changes to the UPS
+     * properties which may impact CurrentPowerStatus
+     *
+     * @param[in]  msg              - Data associated with subscribed signal
+     *
+     */
+    void uPowerChangeEvent(sdbusplus::message::message& msg);
 };
 
 } // namespace manager