network-bridge: default to eth0 if no default gateway.

From: Luke Szymanski <Lukasz.Szymanski@unisys.com>
Signed-off-by: Keir Fraser <keir@xensource.com>

tools/examples/network-bridge

@@ -58,6 +58,7 @@ evalVariables "$@"
 
 netdev=${netdev:-$(ip route list | awk '/^default / { print $NF }' |
 		   sed 's/.* dev //')}
+netdev=${netdev:-eth0}
 bridge=${bridge:-${netdev}}
 antispoof=${antispoof:-no}