adt-setup-vm: Don't use host interface name when building a chroot

E. g. when being called from e.g. vmdebootstrap. Only do that when called from
within a VM. If $root is not /, use eth0 instead, and then mask udev interface
renaming to keep the kernel name. Also, don't touch network configuration at
all if some (apart from lo) already exists.

Closes: #801209