Implement Muen shared memory channels

Muen shared memory channels are an implementation of the SHMStream
Version 2 IPC protocol (shmstream).
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.
hvc_muen: Implement console get operation