hw/hppa/machine: Disable default devices with --nodefaults option

hw/hppa/machine: Disable default devices with --nodefaults option

Recognize the qemu --nodefaults option, which will disable the
following default devices on hppa:
- lsi53c895a SCSI controller,
- artist graphics card,
- LASI 82596 NIC,
- tulip PCI NIC,
- second serial PCI card,
- USB OHCI controller.

Adding this option is very useful to allow manual testing and
debugging of the other possible devices on the command line.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>