[Qemu-devel] Generalize MDIO framework

[Grant Likely]

Hopefully I've responded to everyone's comments here. Edgar, I did end
up fixing up xilinx_axienet, so can you take a look and make sure it is
correct?

Peter, I'm also unsure about the VMState stuff. I think I got it right,
but I've not worked with that code before and I haven't been able to
figure out how to test it with the versatilepb model.

Cheers,
g.