netvsc: delay setup of VF device

When VF device is discovered, delay bring it automatically up in
order to allow userspace to some simple changes (like renaming).

Reported-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>