hvc_muen: Add Muen HVC skeleton

The Muen hypervisor virtual console uses two shared memory channels to
get and put data.
hvc_muen: Implement console put operation

Characters are written to a Muen shared memory channel.
muen-kbd: Implement Muen virtual keyboard driver

Keycode and press state are read from a Muen shared memory stream and
reported as key events to the input subsystem.