Don't adopt the backing store data when reusing images provided by the
user. This will force a backing chain re-probe as users might have
passed in something unexpected in the overlay where our view of the
backing chain would not correspond.

This is done only for inactive snapshots as there we have way less
verification.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>