xmlrpcpp: Add debug log for incoming XML request.

Dump out the entire XML request received which can be useful in
debugging.

BUG: 24335496
Change-Id: If7b57b670a2062829525594d35e5e17514c71991
TEST:
`test_that $host network_WiFi_SimpleConnect.wifi_check11g` &&
adb shell cat /data/shill_xmlrpc_server.log
diff --git a/src/XmlRpcServerConnection.cpp b/src/XmlRpcServerConnection.cpp
index b9d6def..af00f97 100644
--- a/src/XmlRpcServerConnection.cpp
+++ b/src/XmlRpcServerConnection.cpp
@@ -232,6 +232,8 @@
 {
   int offset = 0;   // Number of chars parsed from the request
 
+  XmlRpcUtil::log(5, "XmlRpcServerConnection::parseRequest:\n%s\n", _request.c_str());
+
   std::string methodName = XmlRpcUtil::parseTag(METHODNAME_TAG, _request, &offset);
 
   if (methodName.size() > 0 && XmlRpcUtil::findTag(PARAMS_TAG, _request, &offset))