check if lxc.netdev.link is set for macvlan
check if lxc.netdev.link is set for macvlan

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 <389bd06664b699d49ce291f515b98373073ad5f4@tls.msk.ru>
Signed-off-by: Daniel Lezcano <3211518dd4d87cb3f7ee12cb52e14b7fb2bc0446@fr.ibm.com>

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