scsi: introduce scsi_build_sense

Move more knowledge of sense data format out of hw/scsi/scsi-bus.c
for reusability.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>