CA-78901: Fix for SXM of VMs with snapshots

This change preserves snapshot_of, snapshot_time and is_a_snapshot of snapshot VDIs across SXM. Tested both cross-pool and intra-pool VM.migrate_send, as well as VDI.pool_migrate.

Also tested SXM of a VM with snapshot from xapi based on this branch to a xapi based on master - it works, except the snapshot info is lost, as expected.