KVM nic script: enable interface forwarding

If forwarding is enabled globally this is a no-op. If instead it's
enabled only for some special interfaces where instance traffic has to
go to/comes from (for example a gre tunnel) then it's useful to
explicitely enable it for the instances interfaces as well.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>