Fix uninitialized value issues reported by Coverity:

  Field 'msg.reserved' is uninitialized when calling write().

Fixes: a5bd0580 ("vhost-vdpa: batch updating IOTLB mappings")
Reported-by: Coverity (CID 1432864: UNINIT)
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201028154004.776760-1-philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>