hw/misc/aspeed_hace: Support accumulative mode for direct access mode

hw/misc/aspeed_hace: Support accumulative mode for direct access mode

Enable accumulative mode for direct access mode operations. In direct access
mode, only a single source buffer is used, so the "iovec" count is set to 1.
If "acc_mode" is enabled:
1. Accumulate "total_req_len" with the current request length ("plen").
2. Check for padding and determine whether this is the final request.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250515081008.583578-12-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>