AttributeError at attaching a non-existent volume

When user tries to attach a non-existent volume, AttributeError
exception is thrown in get_data function, where the right behavior
is returning [].

This patch is based in the solution proposed by Matthew D. Wood.

Change-Id: Ib6c5f83845e0056865a8ace82d9e2f4351d4be3e
Closes-Bug: #1288419
Merge "AttributeError at attaching a non-existent volume"