vm-virtio: Restore queues based on used index

On the restore path, using the available and used indexes read from
memory to fill the Queue structure was a mistake. Indeed, the available
index is written from the guest and it reflects the last available index
in the descriptor table. But the driver might have queued a lot of
buffers which have not yet been used by the device. This leads to a
situation where the next_avail from Queue is completely different from
the one we can read from memory.

Instead, the right way to determine the next_avail index that should be
used by the device is by relying on the used index from the memory. This
index represents the correct information we're looking for as it has
been updated before the snapshot to let the guest know the next index to
process.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/transport/mmio.rs

@@ -175,7 +175,7 @@ impl MmioDevice {
             for queue in self.queues.iter_mut() {
                 queue.next_avail = Wrapping(
                     queue
-                        .avail_index_from_memory(&mem)
+                        .used_index_from_memory(&mem)
                         .map_err(Error::QueueRingIndex)?,
                 );
                 queue.next_used = Wrapping(