We are preparing a certain disk source passed in as '@src' so the
individual functions should use that rather than disk->src which
corresponds to the top level element of the chain only.

Without this change TLS and persistent reservations would not work for
backing images of a chain when using -blockdev.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>