xl: allow specifying a default gatewaydev in xl.conf

This adds a new global option in the xl configuration file called
"vif.default.gatewaydev", that is used to specify the default
gatewaydev to use when none is passed in the vif specification.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Tested-by: Ulf Kreutzberg <ulf.kreutzberg@hosteurope.de>
Cc: Ulf Kreutzberg <ulf.kreutzberg@hosteurope.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/examples/xl.conf

@@ -20,3 +20,6 @@
 # if disabled the old behaviour will be used, and hotplug scripts will be
 # launched by udev.
 #run_hotplug_scripts=1
+
+# default gateway device to use with vif-route hotplug script
+#vif.default.gatewaydev="eth0"