partially fix #3424: vzdump: cleanup: wait for active replication

partially fix #3424: vzdump: cleanup: wait for active replication

As replication and backup can happen at the same time, the vzdump
snapshot might be actively used by replication when backup tries
to cleanup, resulting in a not (or only partially) removed snapshot
and locked (snapshot-delete) container.

Wait up to 10 minutes for any ongoing replication. If replication
doesn't finish in time, the fact that there is no attempt to remove
the snapshot means that there's no risk for the container to end up in
a locked state. And the beginning of the next backup will force remove
the left-over snapshot, which will very likely succeed even at the
storage layer, because the replication really should be done by then
(subsequent replications shouldn't matter as they don't need to
re-transfer the vzdump snapshot).

Suggested-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Co-developed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>