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
just didn't force false on start_pause and force, we can define method as:

def start_server(ref, start_paused = false, force = false) if we think its more convenient