engine: Removing XmlRpcStruct

The following patch is removing XmlRpcStruct.
The XmlRpcStruct was wrapper for HahsMap, we don't need any wrapper for
HashMap

Benefits:
1. Simple code
2. Reduced number of created HashMaps and XmlRpcStruct objects (thouthands of objects per second)
3. Less JVM work

Time estimation for patch including tests: 2 hours (I drink a lot coffe)

Change-Id: Id7f5de61b8b0ea31f6fd16beb02d37b524f39879
Signed-off-by: Michael Kublin <mkublin@redhat.com>