Otherwise 2 block_device_mapping entries will be created
in the API cell:
- the first one (created by the API cell) will have close to
no information about the volume (device_name and volume_id are NULL)
- the second one (bubbled up from the compute cell) will contain
all the volume information
The first entry confuses Nova when creating an image since
it won't be able to find the associated volume (NULL) in Cinder.
The compute cell should create it first and propagate it up
to the API cell.