libqos/ahci: track sector size

libqos/ahci: track sector size

It's not always 512, and it does wind up mattering for PIO tranfers,
because this means DRQ blocks are four times as big for ATAPI.
Replace an instance of 2048 with the correct define, too.

This patch by itself winds changing no behavior. fis->count is ignored
for CMD_PACKET, and sect_count only gets used in non-ATAPI cases.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180606190955.20845-2-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>