pci: Eliminate redundant PCIDevice::bus pointer

pci: Eliminate redundant PCIDevice::bus pointer

The bus pointer in PCIDevice is basically redundant with QOM information.
It's always initialized to the qdev_get_parent_bus(), the only difference
is the type.

Therefore this patch eliminates the field, instead creating a pci_get_bus()
helper to do the type mangling to derive it conveniently from the QOM
Device object underneath.

Signed-off-by: David Gibson <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Reviewed-by: Marcel Apfelbaum <[email protected]>
Reviewed-by: Peter Xu <[email protected]>