ipmid: use the default system bus
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 <4fbc0e9805d36da7c621cd4d73f3eb0d5b12b5f3@linux.intel.com>

openbmc/phosphor-host-ipmid,openbmc/phosphor-host-ipmid,openbmc/phosphor-host-ipmid