pci_find_root_bus() takes a domain parameter.  Currently PCI root buses
with domain other than 0 can't be created, so this is more or less a long
winded way of retrieving the main PCI root bus.  Numbered domains don't
actually properly cover the (non x86) possibilities for multiple PCI root
buses, so this patch for now enforces the domain == 0 restriction in other
places to replace pci_find_root_bus() with an explicit
pci_find_primary_bus().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>