pbs: prune: avoid getting all snapshots for group assembly if fixed anyway

pbs: prune: avoid getting all snapshots for group assembly if fixed anyway

If both type and vmid is defined we don't need to list the current
snapshots, we simply can derive the single backup group from that and
let the PBS client handle the rest.

Should be a not so small speedup for most setups using PBS backup and
pruning configured on PVE side, as vzdump calls this separately for
every vmid on backup jobs with multiple guests included.

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