Add method to check if an interface has been initialized

Different capabilities are supported depending on if an interface
has been initialized (e.g. register_property is only available before
initialization and set_property is only available after).

This change adds a method to check if an interface has been initialized
to help determine which capabilities are currently supported.

Tested: Ran a module that uses register_property and set_property based
on this return value and confirmed that the properties are set
accordingly.

Change-Id: Iac39887f5d92981a316d1046f2f3d3ab71d15878
Signed-off-by: Jason M. Bills <jason.m.bills@linux.intel.com>
diff --git a/sdbusplus/asio/object_server.hpp b/sdbusplus/asio/object_server.hpp
index a3b511f..0026530 100644
--- a/sdbusplus/asio/object_server.hpp
+++ b/sdbusplus/asio/object_server.hpp
@@ -697,6 +697,11 @@
         return true;
     }
 
+    bool is_initialized()
+    {
+        return initialized_;
+    }
+
     bool signal_property(const std::string& name)
     {
         if (!initialized_)