qdev: sysbus_get_default must not return a NULL pointer (fix regression)

Every system should have some sort of main system bus,
so sysbus_get_default should always return a valid bus.

Without this patch, at least mipssim and malta no longer
start but raise a null pointer access exception (caused by
commit ec990eb622ad46df5ddcb1e94c418c271894d416).

Cc: Anthony Liguori <anthony@codemonkey.ws>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
