tests/acceptance: bound the size of readline in s390_ccw_virtio

The read binary data as text via a PPM export of the frame buffer
seems a bit sketchy and it did blow up in the real world when the
assertion failed:

  https://gitlab.com/qemu-project/qemu/-/jobs/943183183



However short of cleaning up the test to be more binary focused at
least limit the attempt to dump the whole file as hexified zeros in
the logs.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210105124405.15424-1-alex.bennee@linaro.org>

tests/acceptance/machine_s390_ccw_virtio.py

@@ -241,7 +241,7 @@ def test_s390x_fedora(self):
             self.assertEqual(line, b"1024 768\n")
             line = ppmfile.readline()
             self.assertEqual(line, b"255\n")
-            line = ppmfile.readline()
+            line = ppmfile.readline(256)
             self.assertEqual(line, b"The quick fox jumps over a lazy dog\n")
 
         # Hot-plug a virtio-crypto device and see whether it gets accepted