This patch implements the page response path. The
response is written into the page response ring buffer and then
update header's head index is updated. This path is not used
by this series. It is introduced here as a POC for vSVA/ARM
integration.

Signed-off-by: Eric Auger <eric.auger@redhat.com>

---

v11 -> v12:
- use VFIO_REGION_INFO_CAP_DMA_FAULT_RESPONSE [Shameer]
- fix hot del regression reported and fixed by Shameer