pci: improve PCI bus discovery

When we discovery devices on a PCI BUS, we block bridges, and
we use PCI_SECONDARY_BUS to create a linked list of bridges to
recursively setup new buses. This avoids a second visit of all the
addresses on the bus.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>