qdev: provide qdev_reset_all_fn()

qdev: provide qdev_reset_all_fn()

For TYPE_DEVICE, the dc->reset() function is not called on system resets
yet. Until that is changed, we have to manually register a reset handler.
Let's provide qdev_reset_all_fn(), that can directly be used - just like
the reset handler that is already available for qbus.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>