partially fix #3111: snapshot rollback: improve removing replication snapshots

partially fix #3111: snapshot rollback: improve removing replication snapshots

Get the replicatable volumes from the snapshot config rather than the
current config. And filter those volumes further to those that will
actually be rolled back.

Previously, a volume that only had replication snapshots (e.g. because
it was added after the snapshot was taken, or the vmstate volume)
would lose them.  Then, on the next replication run, such a volume
would lead to an error, because replication tried to do a full sync,
but the target volume still exists.

This is not a complete fix. It is still possible to run into problems:
- by removing the last (non-replication) snapshots after a rollback
  before replication can run once.
- by creating a snapshot and making a rollback before replication can
  run once.

The list of volumes is not required to be sorted for prepare(), but it
is sorted by how foreach_volume() iterates now, so not random.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>