e1000e (82579LM) management mode under KVM/QEMU

We are seeing some issues with an e1000e (82579LM) interface if
the 'management mode' stuff is enabled to allow VNC+ console access.
This is an Ivy bridge cpu, so will be a Panther point (PCH7) chipset.

The 82579LM interface and a second 82574L one are 'passed through'
to a KVM/qemu VM and then bonded together.

The link is only coming up at 10M HDX (sometimes FDX) even after
replugging the cable.
(ping -s 60000 confirms that the reported speed is correct.)

Forcing n-way negotiation with ethtool results in the correct
100M/1G speed.

The failing system is running an ubuntu built 3.11 kernel.
I don't see the same problem on a non-VM with 3.14(ish) kernel
on the same motherboard.
Running a more recent kernel on the VM is a bit tricky.

I can't see any obvious changes since 3.11, but the driver code
has a lot of conditions based on the PCH (etc) type.
I'm not sure whether the VM kernel is using the correct functions.
I can't see any trace that indicates which host controller the
e1000e driver thinks the device is connected to.

Any hints on how to determine what is actually going on?

	David