vhost_iotlb: split out IOTLB initialization

This patch splits out IOTLB initialization to make sure it could be
reused by external modules.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Gautam Dawar <gdawar@xilinx.com>
Message-Id: <20220330180436.24644-9-gdawar@xilinx.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>