docs: Move sections around in pci-addresses.rst

docs: Move sections around in pci-addresses.rst

The section about VFIO devices is kept separate from the rest
because it's less about domain XML and guest OS disagreeing on the
PCI address of a device, and more about which of the two PCI
addresses in the domain XML is even relevant to the guest OS.

The section on zPCI addresses, on the other hand, falls squarely
in the "more complex cases" category, so it should live in the
corresponding section.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>