Re: [RFC v3 25/29] vhost: Add custom IOTLB translations to SVQ

Now every element needs to store the previous address also, so VirtQueue
can consume the elements properly. This adds a little overhead per VQ
element, having to allocate more memory to stash them. As a possible
optimization, this allocation could be avoided if the descriptor is not
a chain but a single one, but this is left undone.

Checking also for vhost_set_iotlb_callback to send used ring remapping.
This is only needed for kernel, and would print an error in case of
vhost devices with its own mapping (vdpa).

This could change for other callback, like checking for
vhost_force_iommu, enable_custom_iommu, or another. Another option could
be to, at least, extract the check of "is map(used, writable) needed?"
in another function. But at the moment just copy the check used in
vhost_dev_start here.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
  hw/virtio/vhost-shadow-virtqueue.c | 134 ++++++++++++++++++++++++++---
  hw/virtio/vhost.c                  |  29 +++++--
  2 files changed, 145 insertions(+), 18 deletions(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index 934d3bb27b..a92da979d1 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c

@@ -10,12 +10,19 @@
  #include "hw/virtio/vhost-shadow-virtqueue.h"
  #include "hw/virtio/vhost.h"
  #include "hw/virtio/virtio-access.h"
+#include "hw/virtio/vhost-iova-tree.h"
  
  #include "standard-headers/linux/vhost_types.h"
  
  #include "qemu/error-report.h"
  #include "qemu/main-loop.h"
  
+typedef struct SVQElement {
+    VirtQueueElement elem;
+    void **in_sg_stash;
+    void **out_sg_stash;