pci: Move VMState registration/unregistration to QOM realize/unrealize

Use the realize and unrealize hooks to register and unregister
vmstate_pcibus respectively.

Relocate some stuff to avoid forward declarations.

Signed-off-by: Bandan Das <bsd@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
[AF: Keep using PCI_BUS() cast macro]
Signed-off-by: Andreas Färber <afaerber@suse.de>
