pci: allow cleanup/unregistration of PCI root buses

pci: allow cleanup/unregistration of PCI root buses

This adds cleanup counterparts to pci_register_root_bus(),
pci_root_bus_new(), and pci_bus_irqs().

These cleanup routines are needed in the case of hotpluggable
PCIHostBridge implementations. Currently we can rely on the
object_unparent()'ing of the PCIHostState recursively unparenting
and cleaning up it's child buses, but we need explicit calls
to also:

  1) remove the PCIHostState from pci_host_bridges global list.
     otherwise, we risk accessing freed memory when we access
     the list later
  2) clean up memory allocated in pci_bus_irqs()

Both are handled outside the context of any particular bus or
host bridge's init/realize functions, making it difficult to
avoid the need for explicit cleanup functions without remodeling
how PCIHostBridges are created. So keep it simple and just add
them for now.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>