Fix creating bdm for failed volume attachment

This commit modifies the reserve_block_device_name method to return the
bdm object, when the corresponding keyword argument is True. This
ensures the correct bdm is destroyed if the attach fails. Currently the
code assumes only one bdm per volume and so retrieving it can cause the
incorrect db entry to be returned.

Change-Id: I22a6db76d2044331d1a846eb4b6d7338c50270e2
Closes-Bug: #1349888
Merge "Fix creating bdm for failed volume attachment"