bhyve/snapshot: switch to nvlist for snapshot requests

Switch to using an nvlist with nvlist_send()/nvlist_recv() to
communicate from bhyvectl(8) to bhyve(8).

The idea is that a bhyve process receives a command with with a set of
arguments. The nvlist here is structured to reflect that premise.

For example, to snapshot the vm, the expected nvlist looks like:

{ cmd=START_CHECKPOINT, filename="filename" }

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D33977