Some guests use a mixture of DMA and non-DMA transfers in combination with the
SATN and stop command to transfer message out phase and command phase bytes to
the target. Prepare for the next commit by adding a maxlen parameter to
get_cmd() to allow partial transfers.

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: <20210304221103.6369-37-mark.cave-ayland@ilande.co.uk>