Commit 16ca234b refactored how the 'shallow' and 'reuse' flags
are accessed but neglected to fix the clearing of 'shallow' in case when
the disk has no backing chain. This means that we'd request a shallow
copy even without backing chain and also a few checks would work wrong.

Fix it by using the extracted variable everywhere.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>