vhost/net: Clear the pending messages when the backend is removed

When the vhost iotlb is used along with a guest virtual iommu
and the guest gets rebooted, some MISS messages may have been
recorded just before the reboot and spuriously executed by
the virtual iommu after the reboot.

As vhost does not have any explicit reset user API,
VHOST_NET_SET_BACKEND looks a reasonable point where to clear
the pending messages, in case the backend is removed.

Export vhost_clear_msg() and call it in vhost_net_set_backend()
when fd == -1.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Suggested-by: Jason Wang <jasowang@redhat.com>
Fixes: 6b1e6cc7

 ("vhost: new device IOTLB API")
Message-Id: <20230117151518.44725-3-eric.auger@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>