count image files on NFS as shared block storage

If instance path is shared between compute nodes (e.g. over NFS) and image
backend uses files placed in the instance directory (e.g. Raw, Qcow2), live
migration should consider block storage of the instances shared.

Change-Id: I913c57152974562765502ab57be63ccb6dfe5208
Closes-Bug: #1351002