When setting the mtu size at the veth creation, the mtu is only set
on one side of the veth tunnel, the one attached to the bridge.

I changed a little the code and added the device_set_mtu function so
it is called after the veth has been created on both side.

That moves the mtu veth specific code inside the veth function creation.

Hopefully this code could be reused later for different future network
configuration (eg. ip tunnel).

The mtu option will be simply ignored in case of macvlan network configuration
because the macvlan network device inherit the mtu of the physical link.
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>