[#62905634] switch Vm class to be immutable on @id
[#62905634] switch Vm class to be immutable on @id

Replace storage of vcloud_attributes with @id.

This removes the possibility of creating Vm objects that are in a variety
of states, and clarifies that this is an abstraction of the actual state in
vCloud Director.

Makes class immutable too, which is a nice benefit.

gds-operations/vcloud-core,UKHomeOffice/vcloud-core,gds-operations/vcloud-core,UKHomeOffice/vcloud-core