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

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

Introduce a new struct, ipc_message - this struct stores the type of
message and a union containing message specific structures for the type
of message being sent.

Reviewed by: grehan
Differential Revision: https://reviews.freebsd.org/D30221