vhost: workaround stale vring base

vhost: workaround stale vring base

When DPDK app crashes (or quits, or gets killed), a restart of DPDK
app would get stale vring base from QEMU. That would break the kernel
virtio net completely, making it non-work any more, unless a driver
reset is done.

So, instead of getting the stale vring base from QEMU, Huawei suggested
we could get a much saner (and may not the most accurate) vring base
from used->idx. That would work because:

- there is a memory barrier between updating used ring entries and
  used->idx. So, even though we crashed at updating the used ring
  entries, it will not cause any issue, as the guest driver will not
  process those stale used entries, for used-idx is not updated yet.

- DPDK process vring in order, that means a crash may just lead some
  packet retransmission for Tx and drop for Rx.

Suggested-by: Huawei Xie <huawei.xie@intel.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Huawei Xie <huawei.xie@intel.com>