send PLDM event msg when state sensor changes

Add the D-Bus to event handler interface to listen all of the state
sensor PDRs and pldm should be able to send up a PLDM event msg when a
D-Bus property changes. the PlatformEventMessage command format is
defined in Table 15 in DSP0248 v1.2.0.

Tested: test with JSON
https://gist.github.com/lxwinspur/6a40abea7330c25e4d49826e890c4be9
The sendEventMsg method is successfully called and the corresponding
message is successfully sent when the D-Bus property value of the sensor
status changes.
print requestMsg Data: 80 02 0a 01 00 00 01 00 01 00 00 00(when calling
sendEventMsg method)

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I9b6d5c1403bfcaa00dbbf478f7d797cc4a40f20d
diff --git a/libpldm/base.h b/libpldm/base.h
index 5109c34..7f87971 100644
--- a/libpldm/base.h
+++ b/libpldm/base.h
@@ -113,6 +113,8 @@
 // Macros for byte-swapping variables in-place
 #define HTOLE32(X) (X = htole32(X))
 #define HTOLE16(X) (X = htole16(X))
+#define LE32TOH(X) (X = le32toh(X))
+#define LE16TOH(X) (X = le16toh(X))
 
 /** @struct pldm_msg
  *