Multi-queue virtio-scsi uses a different scsi_host_template struct.  Add
the .device_alloc field there, too.

Fixes: 25d1d50e
Cc: stable@vger.kernel.org
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>