rtnetlink: add helpers to dump vf information

similar to earlier patches, split out more parts of this function to
better see what is happening and where we assume rtnl is locked.

Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>