Modern way to store <auth> and <encryption> of a <disk> is under
<source>. This was added to mirror how <backingStore> handles these and
in fact they are relevant to the source rather than to any other part of
the disk. Historically we allowed them to be directly under <disk> and
we need to keep compatibility.

This wasn't a problem until introduction of -blockdev in qemu using of
<auth> or <encryption> plainly wouldn't work with backing chains.

Now that it works in backing chains and can be moved back and forth
using snapshots/block-commit we need to ensure that the original
placement is properly kept even if the source changes.

To achieve the above semantics we need to store the preferred placement
with the disk definition rather than the storage source definitions and
also ensure that the modern way is chosen when the VM started with
<source/encryption> only in the backing store.

https://bugzilla.redhat.com/show_bug.cgi?id=1822878Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>