net: replace net_client_init1() netdev whitelist with blacklist

It's cumbersome to keep the whitelist up-to-date.  New netdev backends
should most likely be allowed so a blacklist makes more sense than a
whitelist.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 1432743412-15943-3-git-send-email-stefanha@redhat.com