Reopening a node's backing child needs a bit of special handling because
the "backing" child has different defaults than all other children
(among other things).  Adding filter support here is a bit more
difficult than just using the child access functions.  In fact, we often
have to directly use bs->backing because these functions are about the
"backing" child (which may or may not be the COW backing file).

Signed-off-by: Max Reitz <mreitz@redhat.com>