hardware: enumerate registered sensors in IETFHardware

For the alarm-inventory listing we will need all the sensors that
IETFHardware knows about and can possibly create alarm on them.

Change-Id: I852d90dc76edad92544a6f91860c6076c52b6d95
diff --git a/src/ietf-hardware/IETFHardware.h b/src/ietf-hardware/IETFHardware.h
index e703434..c829721 100644
--- a/src/ietf-hardware/IETFHardware.h
+++ b/src/ietf-hardware/IETFHardware.h
@@ -68,6 +68,7 @@
     }
 
     HardwareInfo process();
+    std::vector<std::string> sensorsXPaths() const;
 
 private:
     velia::Log m_log;