[aoe] Use an AoE config query to identify the target MAC address

[aoe] Use an AoE config query to identify the target MAC address

The AoE spec does not specify that the source MAC address of a
received packet actually matches the MAC address of the AoE target.
In principle an AoE server can respond to an AoE request on any
interface available to it, which may not be an address configured to
accept AoE requests.

This issue is resolved by implementing AoE device discovery.  The
purpose of AoE discovery is to find out which addresses an AoE target
can use for requests.  An AoE configuration command is sent when the
AoE attach is attempted.  The AoE target must respond to that
configuration query from an interface that can accept requests.

Based on a patch from Ryan Thomas <ryan@coraid.com>