tests: use static qga config file

Do not create a leaking temporary file, but use a static file instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
