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

The nvlist for a checkpoint request will now look like:

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

Reviewed by: jhb
Suggested by: jhb
Differential Revision: https://reviews.freebsd.org/D34237

(cherry picked from commit 4379c1da56faa43ecc925e47707a2f51b488614e)