Fix code which finds the default interface if there are static routes.

Fix code which finds the default interface if there are static routes.
The current code will break if there are multiple scope global routes
since the "ip route list default" command doesn't use "default" as a
filter, and will output multiple devices which breaks the rest of the
script.
Also make the vif/xenbr/eth naming consistent, i.e. if your default
network interface is eth1, you get vif1 and xenbr1.

Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>