Add messages to VM model

Add messages keeping most similar to XenServer api, including
parameters names

Some classes were forcing some parameters to XenServer, I just left
them needing to be explicit passed, if we define its better assume
some default values, no problem
hummm got it... what about just keep a dummy parameter, like:

def shutdown_server(ref, stype = false)
  send_deprecated_message unless stype == false

what do you think about this approach, this way we:
1. support the old api
2. warn people about the deprecated parameter

thoughts?