When running the qtests, there are currently a bunch of warnings about
the deprecated sga device during the boot-serial-test. Switch to
"-M graphics=off" to silence these warnings.

Message-Id: <20211220164042.397028-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>