Document bridge config option in default configuration.

This new option was added by Dmitry Nedospasov <dmitry@nedos.net>
in commit c03029e0c6

etc/xen-tools.conf

@@ -169,6 +169,12 @@ image  = sparse   # Specify sparse vs. full disk images.
 #
 # dhcp = 1
 
+#
+# Setup bridge name for host vif. Usefull if you use bridged networking
+# for guests.
+#
+# bridge = xendmz
+#
 
 ##
 # Misc options