replication: prepare: include volumes without snapshots in the result

replication: prepare: include volumes without snapshots in the result

Note that PVE::Storage::volume_snapshot_info() will fail when a volume
does not exist, so no non-existing volume will end up in the result
(prepare() is only called with volumes that should exist).

This makes it possible to detect a volume without snapshots in the
result of prepare(), and as a consequence, replication will now also
fail early in a situation where source and remote volume both exist,
but (at least) one of them doesn't have any snapshots.

Such a situation can happen, for example, by deleting and re-creating
a volume with the same name on the source side without running
replication after deletion.

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