network: suppress NetworkManager from touching inter-vm interfaces (#774)

Those interfaces are configured by qubes scripts (based on xenstore data
filled by qubes core).

network/network-manager-prepare-conf-dir

@@ -8,4 +8,11 @@ if [ -d $NM_CONFIG_DIR -a ! -h $NM_CONFIG_DIR ]; then
     ln -s /rw/config/NM-system-connections $NM_CONFIG_DIR
 fi
 
+# Do not manage xen-provided network devices
+unmanaged_devices=mac:fe:ff:ff:ff:ff:ff
+for mac in `xenstore-ls device/vif | grep mac | cut -d= -f2 | tr -d '" '`; do
+    unmanaged_devices="$unmanaged_devices;mac:$mac"
+done
+sed -i -e "s/^unmanaged-devices=.*/unmanaged-devices=$unmanaged_devices/" /etc/NetworkManager/NetworkManager.conf
+
 exit 0