ipmid: use the default system bus

The documentation says sd_bus_default_system() is preferred over
sd_bus_open_system() because all the calls to sd_bus_default_system()
will use the same resources, whereas the sd_bus_open_system() will
create a new connection and consume more resources for every call.

Change-Id: I0206a43eb2a8dd7c76b8246c9f9640a95b2e473c
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
diff --git a/ipmid.cpp b/ipmid.cpp
index 2729cbb..1e7fd0d 100644
--- a/ipmid.cpp
+++ b/ipmid.cpp
@@ -627,7 +627,7 @@
         }
 
     /* Connect to system bus */
-    r = sd_bus_open_system(&bus);
+    r = sd_bus_default_system(&bus);
     if (r < 0)
     {
         log<level::ERR>("Failed to connect to system bus",