It takes a "struct pci_bus *" argument, so we can alloc a PCI device
on a target PCI bus, and it acquires a reference on the pci_bus.
We use pci_alloc_dev(NULL) to simplify the old alloc_pci_dev(),
and keep it for a while but mark it as __deprecated.
Holding a reference to the pci_bus ensures that referencing
pci_dev->bus is valid as long as the pci_dev is valid.