The ExtendedVolumes post-processing extension used to do N
block_device_mapping queries (one query per instance in a 'nova list').
Instead, do one block_device_mapping query with an IN clause of
instance UUIDs.
Change-Id: I32a1bd0e05a7a938e531d00bedfab23a0bb68538
Partial-Bug: #1416132
Closes-Bug: #1359808