Some properties don't make sense to be configured for every single layer
of the backing chain, but to avoid needing to pass the disk structure we
will copy them to the individual virStorageSource.

Zero detection is applied only for the top layer image, while caching
and iomode for all layers.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>