disallow boot from volume from specifying arbitrary volumes

Fix a vulnerability in volume attachment in nova-volume, affecting the
boot-from-volume feature.  By passing a specific volume ID, an
authenticated user may be able to boot from a volume they don't own,
potentially resulting in full access to that 3rd-party volume.

Fixes bug: 1069904, CVE-2013-0208
Change-Id: I5f7c8d20d3ebf33ce1ce64bf0a8418bd2b5a6411