To be passed to qbus_create_inplace().

Use DEVICE() casts instead of direct parent field access.
Reviewed-by: NWenchao Xia <xiawenc@linux.vnet.ibm.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>