hw/ide/ahci: treat a failed PRDT walk as a PIO transfer failure

ahci_dma_prepare_buf() returns -1 when it cannot build a scatter-gather list, the PRDTL of zero case among them. ahci_pio_transfer() tests the result for truth, so a failure sets has_sglist and the transfer goes ahead against whatever s->sg holds. AHCI 1.3.1 is explicit about the zero case: "If this field is '0', then no data transfer shall occur with the command." Test for a positive byte count instead. A successful walk that yields nothing to transfer is already handled by the size check below. Resolves: https://gitlab.com/qemu-project/qemu/-/issues/4043 Cc: John Snow <jsnow@redhat.com> Cc: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com> Signed-off-by: Denis V. Lunev <den@openvz.org>