Revert "force reload volume and vif object when the association from instance is updated."

This reverts commit 12caffb72b7ee4154ffb65d11c697da854e7fc03.
fix that only scoped volumes will be backed up.
force reload instance association from volume always at validation.
Merge pull request #359 from axsh/fix-halted-instance-shared-vol

Fix validation error from volume model.