Check and sync PSU image when PSU is plugged in

When a PSU is plugged out and in, the service shall check the version
and do update if it is with an old software.
When the PSU is plugged in, the model is not available for a while, so
the service subscribe the model property as well, and only create the
software object when both present and model properties are set.

Tested: With dummy update service, verify on Witherspoon that when PSU
        is plugged out (by setting "Present" property in inventory), the
        software object is removed, and when it's pluggd int (by setting
        "Present" property), the software object is created after the
        model is got, and is upgraded by a newer image stored in BMC
        filesystem.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: Ia7516e5bc9c642158b216036bcddf404157f9204
diff --git a/src/version.hpp b/src/version.hpp
index dbc48d7..e26bb31 100644
--- a/src/version.hpp
+++ b/src/version.hpp
@@ -149,6 +149,12 @@
     /** @brief The temUpdater's erase callback. */
     eraseFunc eraseCallback;
 
+    /** @brief Get the version string. */
+    const std::string& getVersionString() const
+    {
+        return versionStr;
+    }
+
   private:
     /** @brief Persistent sdbusplus DBus bus connection */
     sdbusplus::bus::bus& bus;