conf: try to retrieve mtu from veth
conf: try to retrieve mtu from veth

When the mtu cannot be retrieved from netdev->link try from veth device.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@canonical.com>

caglar10ur/lxc,federicobriata/lxc,ss1h2a3tw/lxc,hfuCN/lxc,terceiro/lxc,GreatFruitOmsk/lxc,federicobriata/lxc,thmo/lxc,ysbnim/lxc,thmo/lxc,cjwatson/lxc,terceiro/lxc,hfuCN/lxc,aeris/lxc,hallyn/lxc,thmo/lxc,cjwatson/lxc,aeris/lxc,cjwatson/lxc,GreatFruitOmsk/lxc,ysbnim/lxc,hallyn/lxc,LynxChaus/lxc,hfuCN/lxc,GreatFruitOmsk/lxc,hallyn/lxc,caglar10ur/lxc,ss1h2a3tw/lxc,ysbnim/lxc,LynxChaus/lxc,LynxChaus/lxc,ss1h2a3tw/lxc,terceiro/lxc,caglar10ur/lxc,federicobriata/lxc,aeris/lxc