tests/functional: avoid accessing log_filename on earlier failures

tests/functional: avoid accessing log_filename on earlier failures

If a failure occurs early in the QemuBaseTest constructor, the
'log_filename' object atttribute may not exist yet. This happens
most notably if the QEMU_TEST_QEMU_BINARY is not set. We can't
initialize 'log_filename' earlier as we use the binary to identify
the architecture which is then used to build the path in which the
logs are stored.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20241121154218.1423005-19-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20241121165806.476008-19-alex.bennee@linaro.org>