Prior to this commit, Vagrant would list all machine snapshots in a flat
list, without showing which snapshots were associated with which guests.
This commit fixes that by placing some separation to make it clear which
snapshots belong to which guests.
Prior to this commit, if a user attempted to use the `vagrant snapshot
save` or `vagrant snapshot list` commands on a vm whose provider did not
support snapshots, it would simply print a warning. This commit changes
that behavior by instead raising an error.
Brian Cain
Mitchell Hashimoto