Adds two helper commands which automate sending and promoting VM
snapshots as "vm mirror" commands.
"vm mirror create" replicates the functionality of "snapshot create" and
"snapshot send", performing both in one single task using an
autogenerated dated snapshot name for automatic cross-cluster
replication.
"vm mirror promote" replicates the functionality of "vm shutdown",
"snapshot create", "snapshot send", "vm start" (remote), and,
optionally, "vm remove", performing in one single task an entire
cross-cluster VM move with or without retaining the copy on the local
cluster (if retained, the local copy becomes a snapshot mirror of the
remote, flipping their statuses).