dfxml.py: Add null-argument test on content_for_run

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
dfxml.py: Add image reference to iterative reader

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
idifference.py: Promote delta namespace to DFXML library

A second application (in development) that doesn't use idifference.py
now uses this namespace reference.

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>