bhyve/snapshot: use a string for 'cmd' in the nvlist

[rew]

Summary

Suggested by: jhb

[rew]

I'm going to iterate on this.

[rew]

• use "checkpoint" for the "cmd" field in the nvlist
• use a boolean flag to indicate a suspend operation for the checkpoint nvlist

The nvlist for a checkpoint request will now look like:

{ cmd="checkpoint", suspend="true/false", filename="afilename" }