Initial implementation of HostEpoch

When host time is set, the diff between the BmcTime and the value is
saved to persistent storage;
When host time is retrieved, return the BmcTime plus the diff as
host's time.

Add the unit test cases for HostEpoch.

Change-Id: Ia55b93bfcba4f226ceaed8491136ea7afda7bd77
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/main.cpp b/main.cpp
index 05ac453..b3f4b57 100644
--- a/main.cpp
+++ b/main.cpp
@@ -2,11 +2,13 @@
 
 #include "config.h"
 #include "bmc_epoch.hpp"
+#include "host_epoch.hpp"
 
 int main()
 {
     auto bus = sdbusplus::bus::new_default();
     phosphor::time::BmcEpoch bmc(bus, OBJPATH_BMC);
+    phosphor::time::HostEpoch host(bus,OBJPATH_HOST);
 
     bus.request_name(BUSNAME);