Fix some warnings by cppcheck

Warning message:

```
dbusSensor.hpp:28:9: performance: Variable 'servName' is assigned in
constructor body. Consider performing initialization in
initialization list. [useInitializationList]
        servName = getService(bus, path, sensorIntf);
        ^
virtualSensor.hpp:69:55: performance: Function parameter 'path'
should be passed by const reference. [passedByValue]
  SensorParam(sdbusplus::bus::bus& bus, std::string path, void* ctx):
                                                      ^
virtualSensor.hpp:271:44: performance: Function parameter
'configFile' should be passed by const reference. [passedByValue]
    Json parseConfigFile(const std::string configFile);
                                           ^
virtualSensor.cpp:334:26: style: Local variable 'objPath' shadows
outer argument [shadowArgument]
   auto objPath = sensorDbusPath + sensorType + "/" + name;
                         ^
virtualSensor.cpp:255:58: note: Shadowed declaration
                                    const std::string& objPath)
                                                         ^
virtualSensor.cpp:334:26: note: Shadow variable
    auto objPath = sensorDbusPath + sensorType + "/" + name;
          ^
virtualSensor.cpp:338:33: style: Local variable 'name' shadows outer
variable [shadowVariable]
    std::string name = j["ParamName"];
                ^
virtualSensor.cpp:330:29: note: Shadowed declaration
    std::string name = desc.value("Name", "");
                ^
virtualSensor.cpp:338:33: note: Shadow variable
   std::string name = j["ParamName"];
               ^
virtualSensor.cpp:649:56: performance: Function parameter
'configFile' should be passed by const reference. [passedByValue]
Json VirtualSensors::parseConfigFile(const std::string configFile)
```

Tested: Verify that there are no such warnings in local CI.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I8afaf35043c97d0ca6bfcbe9d50c847d126f8e2d
diff --git a/virtualSensor.hpp b/virtualSensor.hpp
index cdf1d97..cff46f7 100644
--- a/virtualSensor.hpp
+++ b/virtualSensor.hpp
@@ -66,7 +66,7 @@
      * @param[in] path    - The Dbus path of sensor
      * @param[in] ctx     - sensor context for update
      */
-    SensorParam(sdbusplus::bus::bus& bus, std::string path, void* ctx) :
+    SensorParam(sdbusplus::bus::bus& bus, const std::string& path, void* ctx) :
         dbusSensor(std::make_unique<DbusSensor>(bus, path, ctx)),
         paramType(dbusParam)
     {}
@@ -268,7 +268,7 @@
     /** @brief Get virual sensor config from DBus**/
     ManagedObjectType getObjectsFromDBus();
     /** @brief Parsing virtual sensor config JSON file  */
-    Json parseConfigFile(const std::string configFile);
+    Json parseConfigFile(const std::string& configFile);
 
     /** @brief Matches for virtual sensors */
     std::vector<std::unique_ptr<sdbusplus::bus::match::match>> matches;