gdbstub: Error locations for -gdb Stash away the option argument with add_device_config(), so we still have its location when we get around to parsing it. This doesn't improve any messages I can see just yet, but that'll change shortly. Signed-off-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>