Include image block device maps in info

In cases for example soft/hard reboot, we were just getting volume and
snapshot block device maps. For block devices with a source type image,
Nova would fall back on a local disk location and fail.

This change allows us to include block devices with a source of an
image, so that we correctly get the right the location of the block
device.

Closes-Bug: #1245719
Change-Id: I1f726293e85183d4b84c05b635a7c606a092992f
(cherry picked from commit 85d0ace169be513c30b09e13a35fa7d912f5b380)
Merge "Include image block device maps in info" into stable/havana