vdpa: add bind_mm/unbind_mm callbacks

These new optional callbacks is used to bind/unbind the device to
a specific address space so the vDPA framework can use VA when
these callbacks are implemented.

Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20230321154228.182769-2-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>