hw/scsi: Rename SCSIRequest::resid as 'residual'

The 'resid' field is slightly confusing and could be
interpreted as some ID. Rename it as 'residual' which
is clearer to review. No logical change.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20220111184309.28637-8-f4bug@amsat.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>