I think we are getting a little out of hand here, and we are mixing up concepts
:).
There are lots of things IPMI *can* do (including serial access, VGA snooping,
LAN access, etc.) but I don't see any value it that. The main thing here is to
emulate the interface to the guest. OOB management is really more appropriately
handled with libvirt. How the BMC integrates into the hardware varies a *lot*
between systems, but it's really kind of irrelevant. (Well, almost irrelevant,
BMCs can provide a direct I2C messaging capability, and that may matter.)
A guest can have one (or more) of a number of interfaces (that are all fairly
bad, unfortunately). The standard ones are called "KCS", "BT" and "SMIC" and
they generally are directly on the ISA bus, but are in memory on non-x86 boxes
(and on some x86 boxes) and sometimes on the PCI bus. Some systems also have
interfaces over I2C, but that hasn't really caught on. Others have interfaces
over serial ports, and that unfortunately has caught on in the ATCA world. And
there are at least 3 different basic types of serial port interfaces with
sub-variants :(. I'm not sure what the USB rndis device is, but I'll look at it.
But there is no IPMI over USB.