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
Good :clap: 

I was hoping to get to this and start doing it all over the code base since everything in here is documented in the [api](http://docs.vmd.citrix.com/XenServer/6.2.0/1.0/en_gb/api/),