attach & detach volumes apis for compute

Add portable storage attach/detach function.
Change-Id: I1a7abcfc83f581601c730d93440dca7d447a53a0
Instead of using hard-coded type ids, I would use keyNames of the diskimage type.

```python
		vg_client = client['Virtual_Guest']
		block_devices = vg_client.getBlockDevices(mask='id, diskImage.type',
										  	      id=1234)

		vols = [format_volume_attachment(vol['diskImageId'], instance_id, '')
                for vol in vols
                if vol['diskImage']['type']['keyName'] != 'SWAP']
```