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
so here I'm gonna change:

def start_server(ref, start_paused = false, force = false)

so we keep support and if someone wanna pass different parameter, will be fine too... ok?