Fix uninitialized value issues reported by Coverity:

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

While the 'struct vhost_msg' does not have a 'reserved' field,
we still initialize it to have the two parts of the function
consistent.

Reported-by: Coverity (CID 1432864: UNINIT)
Fixes: c471ad0e ("vhost_net: device IOTLB support")
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201103063541.2463363-1-philmd@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>