tests/avocado: Fix console data loss

tests/avocado: Fix console data loss

Occasionally some avocado tests will fail waiting for console line
despite the machine running correctly. Console data goes missing, as can
be seen in the console log. This is due to _console_interaction calling
makefile() on the console socket each time it is invoked, which must be
losing old buffer contents when going out of scope.

It is not enough to makefile() with buffered=0. That helps significantly
but data loss is still possible. My guess is that readline() has a line
buffer even when the file is in unbuffered mode, that can eat data.

Fix this by providing a console file that persists for the life of the
console.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: "Daniel P. Berrangé" <berrange@redhat.com>
Message-Id: <20230912131340.405619-1-npiggin@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: John Snow <jsnow@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230914155422.426639-9-alex.bennee@linaro.org>