test: dbus: passive read interface

Added basic unit-tests and added a factory for creating the
DbusPassive read interface so that it can be nicely error checked.  This
is handled via a valid type check where the only valid types are 'fan'
and 'temp'.

Change-Id: I558ed09bf509d26f20c6e431bb0789074e9aa841
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/dbus/util.hpp b/dbus/util.hpp
index e858c0c..44e137c 100644
--- a/dbus/util.hpp
+++ b/dbus/util.hpp
@@ -58,3 +58,4 @@
 
 std::string GetSensorPath(const std::string& type, const std::string& id);
 std::string GetMatch(const std::string& type, const std::string& id);
+bool ValidType(const std::string& type);