vringh: support VA with iotlb

vDPA supports the possibility to use user VA in the iotlb messages.
So, let's add support for user VA in vringh to use it in the vDPA
simulators.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20230321154228.182769-5-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>