This is a step towards sending messages (other than suspend/resume) from
bhyvectl to bhyve.

Define a new struct, ipc_message - this struct stores the type of
message and message specific data that is sent from bhyvectl to bhyve.

To define a new message:

1. add type of message to enum ipc_opcode
2. add message specific data to the ipc_message union
3. setup ipc_message for the new message on the bhyvctl end
4. handle the new message on the bhyve end