#60 checkpoint and xml include fails

[Mark Lentczner]

If you have XML media that contains includes (so the whole tree
isn't in just one file) then checkpointing fails when you change an
object that resides in an included XML file.

For example: If you have the structure:

home.xml
home.d/
foo.xml
bar.xml

If you change an object that resides in foo.xml, then when we
checkpoint, we write home-new.xml, and then try to write a new
foo.xml under home-new.d. Only this directory doesn't exist.