Since Ubuntu 17.10 Artful, Ubuntu has decided to use netplan instead
of ifupdown.
40-setup-networking-deb-netplan has been contributed by Arno and Peter.
(see https://github.com/xen-tools/xen-tools/issues/51#issuecomment-412019510)
Closesxen-tools/xen-tools#51.
Since we have to make a variant of hooks/karmic/ as hook/artful/,
rename hooks/karmic/80-install-kernel (which previously was unique to
hooks/karmic/) to hooks/common/80-install-kernel-ubuntu and add
symlinks to it from hooks/karmic/ and hook/artful/.