bhyve/snapshot: provide a way to send other messages/data to bhyve

[rew]

Summary

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

[rew]

ping, anyone interested in reviewing?

[rew]

closed in commit d4870e3a7256704905655e997b37a866024c2894

The differential revision link in the commit message is the wrong one, my mistake.