pci: Move bridge data structures from pci_bus.h to pci_bridge.h

pci: Move bridge data structures from pci_bus.h to pci_bridge.h

include/hw/pci/pci_bus.h contains several data structures related to PCI
bridges that aren't needed by most users of pci_bus.h.  We already have
a pci_bridge.h, so move them there.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>