tests/qtest/cdrom: Add more s390x-related boot tests

tests/qtest/cdrom: Add more s390x-related boot tests

Let's add two new tests:

1) Booting with "bootindex" is the architected default behavior on the
s390x target, so we should have at least one test that is using the
"bootindex" property.

2) The s390-ccw bios used to fail when other unbootable devices have
been specified before the bootable device (without "bootindex"). Now
that the s390-ccw bios is a little bit smarter here, we should test
this scenario, too, to avoid regressions.

Message-Id: <20200806105349.632-10-thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>