datastore: move list_backup_groups into Datastore impl

datastore: move list_backup_groups into Datastore impl

Having that as static method in BackupInfo makes zero sense and just
complicates call sites, which need to extract the base_path from the
store manually upfront.

Mark old fn as deprecated so that we can do the move in a separate
step.

It's also planned to add an Iterator impl for this to allow more
efficient usage in the future.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>