qbus_reset_all_fn was registered twice, so a lot of device reset
functions were also called twice when QEMU started.
Which was introduced by 80376c3f
This patch fixes it by making the main_system_bus creation not register
reset handler.

Cc: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: NIsaku Yamahata <yamahata@valinux.co.jp>
Tested-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>