tools: propogate MTU to vif frontends (backporting)

[James Dingwall <james-xen@xxxxxxxxxxxxxx>]

Hi,

I've been backporting this series to xen 4.14 and everything relating to the
backend seems to be working well.  For the frontend I can see the mtu value
published to xenstore but it does't appear to be consumed to set the matching
mtu in the guest.

https://lists.xenproject.org/archives/html/xen-devel/2020-08/msg00458.html

Is the expected solution a custom script running in the guest to make the
necessary change or have I missed something in how this is supposed to
operate?

Thanks,
James