Prepare for starting a replication job. This is called on the target
node before replication starts. This call is for internal use, and
return a JSON object on stdout. The method first test if VM <vmid>
reside on the local node. If so, stop immediately. After that the
method scans all volume IDs for snapshots, and removes all replications
snapshots with timestamps different than <last_sync>. It also removes
any unused volumes.
Returns a hash with boolean markers for all volumes with existing
replication snapshots.