libvmm: clean up vmmapi.h

struct checkpoint_op, enum checkpoint_opcodes, and
MAX_SNAPSHOT_VMNAME are not vmm specific, move them out of the vmmapi
header.

They are used for the save/restore functionality that bhyve(8)
provides and are better suited in usr.sbin/bhyve/snapshot.h

Since bhyvectl(8) requires these, the Makefile for bhyvectl has been
modified to include usr.sbin/bhyve/snapshot.h

Reviewed by: kevans, grehan
Differential Revision: https://reviews.freebsd.org/D28410