virtio: add context flag to find vqs

Allows maintaining extra context per vq.  For ease of use, passing in
NULL is legal and disables the feature for all vqs.

Includes fixes by Christian for s390, acked by Cornelia.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/linux/virtio_ring.h

@@ -71,6 +71,7 @@ struct virtqueue *vring_create_virtqueue(unsigned int index,
 					 struct virtio_device *vdev,
 					 bool weak_barriers,
 					 bool may_reduce_num,
+					 bool ctx,
 					 bool (*notify)(struct virtqueue *vq),
 					 void (*callback)(struct virtqueue *vq),
 					 const char *name);
@@ -80,6 +81,7 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
 					struct vring vring,
 					struct virtio_device *vdev,
 					bool weak_barriers,
+					bool ctx,
 					bool (*notify)(struct virtqueue *),
 					void (*callback)(struct virtqueue *),
 					const char *name);
@@ -93,6 +95,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
 				      unsigned int vring_align,
 				      struct virtio_device *vdev,
 				      bool weak_barriers,
+				      bool ctx,
 				      void *pages,
 				      bool (*notify)(struct virtqueue *vq),
 				      void (*callback)(struct virtqueue *vq),