i440fx-pcihost: Use QOM realize for i440fx-pcihost

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
q35: Use type-safe cast instead of direct access of parent dev

And remove variables if possible.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
[AF: Converted remaining access and renamed to parent_obj]
Signed-off-by: Andreas Färber <afaerber@suse.de>