pci_bridge: use parent bus's address space

The switch to the new memory API caused the following problem:

The pci device may call pci_register_bar() to use PCI bus's address
space.  But we don't init PCI bus's address space if it is not bus
0.  A crash was reported:
http://lists.gnu.org/archive/html/qemu-devel/2011-08/msg02243.html

More work will be needed to make bridge filtering work correctly
with the memory API.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
