virtio-scsi: forward scsibus for virtio-scsi-pci.

virtio-scsi: forward scsibus for virtio-scsi-pci.

This fix a bug with scsi hotplug on virtio-scsi-pci:

As virtio-scsi-pci doesn't have any scsi bus, we need to forward scsi-hot-add
to the virtio-scsi-device plugged on the virtio-bus.

Cc: qemu-stable@nongnu.org
Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>