The ESP device already keeps track of the remaining bytes left to transfer via
its TC (transfer counter) register which is decremented for each byte that
is transferred across the SCSI bus.

Switch the transfer logic to use the value of TC instead of dma_left and then
remove dma_left completely, adding logic to the vmstate_esp post_load() function
to transfer the old dma_left value to the TC register during migration from
older versions.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210304221103.6369-14-mark.cave-ayland@ilande.co.uk>