Ensure that lxc.netdev.link is specified for macvlan interfaces,
since it's required.

While at it, simplify logic in instanciate_macvlan():
remove unnecessary-complicating goto statements (we only
need to perform a cleanup in one place)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>