[PATCH] tests/acceptance: bound the size of readline in s390_ccw_virtio

["Alex Bennée" <alex.bennee@linaro.org>]

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>
---
 tests/acceptance/machine_s390_ccw_virtio.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/acceptance/machine_s390_ccw_virtio.py b/tests/acceptance/machine_s390_ccw_virtio.py
index 0f81af9950..5141d6abb2 100644
--- a/tests/acceptance/machine_s390_ccw_virtio.py
+++ b/tests/acceptance/machine_s390_ccw_virtio.py
@@ -241,7 +241,7 @@ class S390CCWVirtioMachine(Test):
             self.assertEqual(line, b"1024 768\n")
             line = ppmfile.readline()
             self.assertEqual(line, b"255\n")
-            line = ppmfile.readline()
+            line = ppmfile.readline(size=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
-- 
2.20.1