tests/acceptance: bound the size of readline in s390_ccw_virtio

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 <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Willian Rampazzo <[email protected]>
Acked-by: Halil Pasic <[email protected]>
Acked-by: Thomas Huth <[email protected]>
Message-Id: <20210105124405[email protected]>