The Management Engine Interface, also known as HECI, is the host's path
to the Management Engine firmware, which arbitrates platform power
management and, on later generations, carries traffic to the graphics
system controller on Intel GPUs.
Bring up the circular buffer in BAR0, complete the host bus message
handshake, enumerate the clients the firmware exposes, and query the
firmware version from the MKHI client. Results are exported through
sysctl. One transaction is in flight at a time; there is no character
device and no client bus.
Firmware state does not survive suspend, so resume re-runs the handshake
and re-enumerates.
Tested on: amd64, Intel 6 Series/C200 (0x1c3a), firmware 7.0.1.1205