KVM nic script: use routed link as table

In order to be able to maintain the node network standard routing
untouched while routing instance traffic through a different dedicated
interface (eg: a gre tunnel) we need to specify the instance routing
path inside a separate table, which will also contain different default
gateway values. This patch fixes the kvm default network script to use
the nic link, if present, as a routing table to use instead of the
default one.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>