In the latest udev world, udev generates predictable device names using
firmware info and/or pci buss info (i.e., eno1 or enps4f0).  So, we now
try to run dhclient only on real ethernet devices (i.e., eth*, en*,
sl*).  There are other kinds of ethernet devices (i.e. wireless, wl*,
ww*) or virtual devices, but we don't care about finding the control net
on those.  Might need to add another device name prefix for PV devices
in Xen guests... we'll see.