qemu.py: use os.path.null instead of /dev/null

For increased portability, let's use os.path.devnull.

Signed-off-by: Amador Pahim <apahim@redhat.com>
Message-Id: <20170901112829.2571-4-apahim@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>