pci: pci bridge related clean up.

- fix bridge prefetchable memory accesser to check 64bit or not.
- use pcibus_t consistently instead mixing pcibus_t and uint64_t.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
