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. 



```
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'
```