Don't allow empty or 0 volume size for images

When specifying the block device that has source_type 'image' and
destination_type 'volume', we should confirm that the volume size is
specified, as Cinder won't be able to guess it, and will reject the
request, thus erroring out the instance.

Closes-bug:#1224469
Change-Id: I87ece3be1a5e9d6b229717f8025ce542707007c4
Merge "Don't allow empty or 0 volume size for images"