tests/boot-sector: Add magic bytes to s390x boot code header

tests/boot-sector: Add magic bytes to s390x boot code header

We're going to use the s390x boot code for testing CD-ROM booting.
But the ISO loader of the s390-ccw bios is a little bit more picky
than the network loader and expects some magic bytes in the header
of the file (see linux_s390_magic in pc-bios/s390-ccw/bootmap.c), so
we've got to add them in our boot code here, too.

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Acked-By: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>