Add interface exposing utility functions

Add utility functions enabling easy method calls on sdbusplus interface
binding objects.

Change-Id: Ie1d01f82604406705869fea0ec19d84c95d90474
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/manager.cpp b/manager.cpp
index b9e8ace..d3e2973 100644
--- a/manager.cpp
+++ b/manager.cpp
@@ -183,6 +183,31 @@
     _refs.erase(_root + p);
 }
 
+details::holder::Base& Manager::getInterfaceHolder(
+        const char *path, const char *interface)
+{
+    return const_cast<const Manager *>(
+            this)->getInterfaceHolder(path, interface);
+}
+
+details::holder::Base& Manager::getInterfaceHolder(
+        const char *path, const char *interface) const
+{
+    std::string p{path};
+    auto oit = _refs.find(_root + p);
+    if(oit == _refs.end())
+        throw std::runtime_error(
+                _root + p + " was not found");
+
+    auto &obj = oit->second;
+    auto iit = obj.find(interface);
+    if(iit == obj.end())
+        throw std::runtime_error(
+                "interface was not found");
+
+    return *iit->second;
+}
+
 } // namespace manager
 } // namespace inventory
 } // namespace phosphor