References: bsc#1172356
The bug is that virt-manager reports a failure when in fact 
the host and guest have added the network interface. The Xen
scripts are failing with an error when in fact that command
is succeeding.

The 'ip' commands seem to abort the script due to a 'set -e' in
xen-script-common.sh with what appears to be an error condition.
However, the command actually succeeds when checked from the
host console or also by inserting a sleep before each ip command
and executing it manually at the command line. This seems to be
an artifact of using 'set -e' everywhere.

Index: xen-4.15.0-testing/tools/hotplug/Linux/xen-network-common.sh
===================================================================
--- xen-4.15.0-testing.orig/tools/hotplug/Linux/xen-network-common.sh
+++ xen-4.15.0-testing/tools/hotplug/Linux/xen-network-common.sh
@@ -90,7 +90,7 @@ _setup_bridge_port() {
     local virtual="$2"
 
     # take interface down ...
-    ip link set dev ${dev} down
+    (ip link set dev ${dev} down || true)
 
     if [ $virtual -ne 0 ] ; then
         # Initialise a dummy MAC address. We choose the numerically
@@ -101,7 +101,7 @@ _setup_bridge_port() {
     fi
 
     # ... and configure it
-    ip address flush dev ${dev}
+    (ip address flush dev ${dev} || true)
 }
 
 setup_physical_bridge_port() {
@@ -136,15 +136,15 @@ add_to_bridge () {
     if [ ! -e "/sys/class/net/${bridge}/brif/${dev}" ]; then
         log debug "adding $dev to bridge $bridge"
         if which brctl >&/dev/null; then
-            brctl addif ${bridge} ${dev}
+            (brctl addif ${bridge} ${dev} || true)
         else
-            ip link set ${dev} master ${bridge}
+            (ip link set ${dev} master ${bridge} || true)
         fi
     else
         log debug "$dev already on bridge $bridge"
     fi
 
-    ip link set dev ${dev} up
+    (ip link set dev ${dev} up || true)
 }
 
 remove_from_bridge () {