Fix libvnode_xen to synch with systemd-networkd
"I discovered that the Canuks (cnuc-*) are DHCPing on the order of every minute."
"I think that systemd-networkd is unhappy that eth0 got moved to the xenbr0 bridge and does not have an address... because /var/run/systemd/network/eth0.network says that it should. So it is trying to drive the configuration to a proper place. We need to patch libvnode_xen to be aware of systemd-networkd, and in the one-time control net/bridge setup, to whack the /var/run/systemd/networkd conf files (which were generated at boot by the emulab-networkd udev helper) to match the real, bridged control net setup, and restart systemd-networkd."