DTD unpack_args should return the device pointer

[abouteiller]

Original report by George Bosilca (Bitbucket: bosilca, GitHub: bosilca).

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After the merge of #491 the users are required to call parsec_dtd_get_dev_ptr to convert the data copy pointer provided by unpack_args into the copy on the target device. We need to facilitate this, such that all data copies returned by the unpack_args will point directly to the device memory.