tools/hotplug: fix invalid frontend path for set_mtu

Hi,

This is a resubmission of patches previously discussed in this thread:

https://lists.xen.org/archives/html/xen-devel/2022-04/msg02209.html

It appears they fell down a crack after review and never made it to git.
This is a fix for reading a custom interface mtu from xenstore when the
interface is given a non-default name along with an adjustment to the
check that the value read is >=68.

Thanks,
James