Before, a veth device pair required a link which was treated as
a bridge device.  Code crashed if there was no lxc.network.link
specified.  Fix that by allowing lxc.network.link to be unset
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>