hw/pci: inform bios if the system has extra pci root buses

The bios looks for 'etc/extra-pci-roots' to decide if
is going to scan further buses after bus 0 tree.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>