xl: add vif.default.bridge

This is a replacement for defaultbridge xl.conf option. The now
deprecated defaultbridge is still supported.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/examples/xl.conf

@@ -23,3 +23,6 @@
 
 # default gateway device to use with vif-route hotplug script
 #vif.default.gatewaydev="eth0"
+
+# default bridge device to use with vif-bridge hotplug scripts
+#vif.default.bridge="xenbr0"