qga: Add initial OpenBSD and NetBSD support

Signed-off-by: Brad Smith <brad@comstyle.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>