pci: Rename root bus initialization functions for clarity

pci: Rename root bus initialization functions for clarity

pci_bus_init(), pci_bus_new_inplace(), pci_bus_new() and pci_register_bus()
are misleadingly named.  They're not used for initializing *any* PCI bus,
but only for a root PCI bus.

Non-root buses - i.e. ones under a logical PCI to PCI bridge - are instead
created with a direct qbus_create_inplace() (see pci_bridge_initfn()).

This patch renames the functions to make it clear they're only used for
a root bus.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>