pci: introduce struct pci_dev

To extend current PCI framework, we need a per-device struct to store
device specific information. Time to have a pci_dev struct. Most of the
current PCI APIs are converted to use this pci_dev object as the first
argument. Currently it only contains one field "bdf", which is the bdf
of current device.

For a few APIs like pci_config_*() ops or pci_find_dev(), I kept the old
interface (use PCI BDF value rather than "struct pci_dev") since they
can be used in a open context that without any specific PCI device.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>