Documentation for ebssurrogate states that all of the devices in
`launch_block_device_mappings` are snapshotted and included in the
image. In fact, only the device that was designated as the root
device was snapshotted. This patch modifies the builder to create
snapshots of all the devices and include them in the image. This
allows creating images with separate filesystems preconfigured,
rather than having to add volumes to `ami_block_device_mappings`
and configure them after boot.
Joseph Wright