dfxml.py: Check for name property

xmlfile is not guaranteed to have ".name".  For instance, in the unit
tests, an IO buffer is used instead of a file.

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Objects.py: Add property definition functions

FileObject was missing the .sha1 and .md5 property functions.  These
may be necessary later for dealing with upper/lowercasing, if that is
ever decided to be one and one way.

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