pci: Return PCI_INTX_DISABLED when no bus INTx routing support

Rather than assert, simply return PCI_INTX_DISABLED when we don't
have a pci_route_irq_fn.  PIIX already returns DISABLED for an
invalid pin, so users already deal with this state.  Users of this
interface should only be acting on an ENABLED or INVERTED return
value (though we really have no support for INVERTED).  Also
complain loudly when we hit this so we don't forget it's missing.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
