[Xenial Regression SRU] Fix "Cannot set MTU higher than 1500 in Xen instance"

[Stefan Bader]

BugLink: https://bugs.launchpad.net/bugs/1781413

The latest Xenial update has broken MTU functionality in Xen: specifically,
setting MTUs larger than 1500 fails. This prevents Jumbo Frames and other
features which require larger than 1500 byte MTUs from being used. This can
lead to a failure to sync/connect to other components in the cluster/cloud
which expect higher MTUs and result in unavailable services.

The fix is to cherry-pick 2 patches from upstream. I have not searched to
deeply but those seem not pending on upstream stable.

-Stefan