vhost-vdpa: Do not send empty IOTLB update batches

vhost-vdpa: Do not send empty IOTLB update batches

With the introduction of the batch hinting, meaningless batches can be
created with no IOTLB updates if the memory region was skipped by
vhost_vdpa_listener_skipped_section. This is the case of host notifiers
memory regions, device un/realize, and others. This causes the vdpa
device to receive dma mapping settings with no changes, a possibly
expensive operation for nothing.

To avoid that, VHOST_IOTLB_BATCH_BEGIN hint is delayed until we have a
meaningful (not skipped section) mapping or unmapping operation, and
VHOST_IOTLB_BATCH_END is not written unless at least one of _UPDATE /
_INVALIDATE has been issued.

v3:
  * Use a bool instead of a counter avoiding potential number wrapping
  * Fix bad check on _commit
  * Move VHOST_BACKEND_F_IOTLB_BATCH check to
    vhost_vdpa_iotlb_batch_begin_once

v2 (from RFC):
  * Rename misleading name
  * Abstract start batching function for listener_add/del

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20210812140933.226288-1-eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>