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
I think we might set a default value for now so we won't break the old api. After we successfully split `Xenserver` from the main fog gem we can start making those moves.