Add MTU as a Route property

[aojea]

I think this comment from @joestringer is spot on cilium/cilium#23711 (comment)

One possible solution is to teach kubevirt about the route (+MTU) when configuring the devices. By the looks, the CNI spec doesn't encode this today in its routes, but maybe it should encode MTU in the routes there.

cni/pkg/types/types.go

Lines 131 to 134 in e255525

	type Route struct {
	Dst net.IPNet
	GW net.IP
	}

For the cases of CNIs that does not use the traditional way of changing the network interface and use routeMTU to handle the discrepancies between interfaces, it will be useful if the plugins will be able to set the mtu directly from the CNI configuration.

[aojea]

/assign @squeed

[aojea]

previous work from @mccv1r0 #831