tests/acceptance: Ignore binary data sent on serial console

tests/acceptance: Ignore binary data sent on serial console

If a guest sends binary data on the serial console, we get:

 File "tests/acceptance/avocado_qemu/__init__.py", line 92,
   in _console_interaction msg = console.readline().strip()
 File "/usr/lib64/python3.8/codecs.py", line 322,
   in decode (result, consumed) = self._buffer_decode(data, self.errors, final)
 UnicodeDecodeError: 'utf-8' codec can't decode byte 0xa9 in position 2: invalid start byte

Since we use the console with readline(), fix it the easiest
way possible: ignore binary data (all current tests compare
text string anyway).

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210515134555.307404-2-f4bug@amsat.org>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Tested-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>