Factor out getPowerState() helper function

This same pattern had been open-coded in many of the sensor daemons;
let's reduce the duplication a bit.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Change-Id: I8556af30fdcd53cdf06e5a4911f9152d3265c7af
diff --git a/src/IpmbSensor.cpp b/src/IpmbSensor.cpp
index 8d4dfaa..69c1e46 100644
--- a/src/IpmbSensor.cpp
+++ b/src/IpmbSensor.cpp
@@ -475,15 +475,7 @@
         offsetVal = std::visit(VariantToDoubleVisitor(), findOffsetVal->second);
     }
 
-    auto findPowerState = entry.find("PowerState");
-
-    if (findPowerState != entry.end())
-    {
-        std::string powerState =
-            std::visit(VariantToStringVisitor(), findPowerState->second);
-
-        setReadState(powerState, readState);
-    }
+    readState = getPowerState(entry);
 }
 
 void createSensors(