tests/avocado/machine_s390_ccw_virtio: Adapt test to new default resolution

tests/avocado/machine_s390_ccw_virtio: Adapt test to new default resolution

QEMU's default screen resolution recently changed to 1280x800, so the
resolution in the screen shot header changed of course, too.

Fixes: de72c4b7cd ("edid: set default resolution to 1280x800 (WXGA)")
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220221101933.307525-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>