Fix unexpected validation issue of volume model

It seems that the model class validation runs for unexpected case due to the association model object caching. 


API returns following trace when calls detach volume operation for halted instance.

```
Sequel::ValidationFailed - guest_device_name require to have device name:
/opt/axsh/wakame-vdc/dcmgr/vendor/bundle/ruby/2.0.0/gems/sequel-3.47.0/lib/sequel/model/base.rb:1308:in `save'
/opt/axsh/wakame-vdc/dcmgr/vendor/bundle/ruby/2.0.0/gems/sequel-3.47.0/lib/sequel/model/associations.rb:1401:in `block in def_one_to_many'
/opt/axsh/wakame-vdc/dcmgr/vendor/bundle/ruby/2.0.0/gems/sequel-3.47.0/lib/sequel/model/associations.rb:1638:in `remove_associated_object'
/opt/axsh/wakame-vdc/dcmgr/vendor/bundle/ruby/2.0.0/gems/sequel-3.47.0/lib/sequel/model/associations.rb:1423:in `block in def_remove_methods'
/opt/axsh/wakame-vdc/dcmgr/lib/dcmgr/models/volume.rb:234:in `detach_from_instance'
/opt/axsh/wak
ame-vdc/dcmgr/lib/dcmgr/endpoints/12.03/volumes.rb:240:in `block (2 levels) in load_namespace'
```