Phantom device mapper volume usage in Proxmox: logical volume is used by another device

Virtual disk migration on Proxmox:beaubourg from beaubourg-ssd to Ceph fails with this kind of error:

volume deactivation failed: beaubourg-ssd:vm-107-disk-0 at /usr/share/perl5/PVE/Storage.pm line 1042.```

It has been observed with vm-102-disk-0 and vm-107-disk-0 so far.

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*Migrated from T1501 ([view on Phabricator](https://forge.softwareheritage.org/T1501))*