Correct the MAC address format in Redfish

The MAC address format in Redfish requires 2 character per octet.
ether_ntoa() converts the MAC address to string but omits the
leading zeros which results in one character per octet.
Changed the conversion to display two characters for each octet.

Tested:
sit0 interface:
  "MACAddress": "00:00:00:00:00:00",

eth0 interface:
  "MACAddress": "00:bd:4f:29:0f:d2",

Change-Id: I7660616d4026c4237964be048fea10d7b0d3d18d
Signed-off-by: Karthick Sundarrajan <karthick.sundarrajan@intel.com>
diff --git a/test/test_util.cpp b/test/test_util.cpp
index 4bc9d5b..89723f5 100644
--- a/test/test_util.cpp
+++ b/test/test_util.cpp
@@ -324,6 +324,10 @@
 {
     EXPECT_EQ("11:22:33:44:55:66",
               toString({0x11, 0x22, 0x33, 0x44, 0x55, 0x66}));
+    EXPECT_EQ("01:02:03:04:05:67",
+              toString({0x01, 0x02, 0x03, 0x04, 0x05, 0x67}));
+    EXPECT_EQ("00:00:00:00:00:00",
+              toString({0x00, 0x00, 0x00, 0x00, 0x00, 0x00}));
 }
 
 TEST(MacIsEmpty, True)