qbus: register reset handler for qbus whose parent is NULL

Stefan Weil reported the regression caused by
ec990eb622ad46df5ddcb1e94c418c271894d416 as follows

> The second regression also occurs with MIPS malta.
> Networking no longer works with the default pcnet nic.
>
> This is caused because the reset function for pcnet is no
> longer called during system boot. The result in an invalid
> mac address (all zero) and a non-working nic.
>
> For this second regression I still have no simple solution.
> Of course mips_malta.c should be converted to qdev which
> would fix both problems (but only for malta system emulation).

The issue is, it is assumed that all qbuses, qdeves are under
main_system_bus. But there are qbuses whose parent is NULL. So it
is necessary to trigger reset for those qbuses.
(On the other hand, if NULL is passed to qdev_create(), its parent bus
is main_system_bus.)
Ideally those buses should be moved under bus controller
device which is qdev. But it's not done yet.
So register qbus reset handler for qbus whose parent is NULL.

Reported-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
