Since Ceph version Infernalis (9.2.0) the new fast-diff mechanism
of RBD allows for querying actual volume usage.

Prior to this version there was no easy and fast way to query how
much allocation a RBD volume had inside a Ceph cluster.

To use the fast-diff feature it needs to be enabled per RBD image
and is only supported by Ceph cluster running version Infernalis
(9.2.0) or newer.

Without the fast-diff feature enabled libvirt will report an allocation
identical to the image capacity. This is how libvirt behaves currently.

'virsh vol-info rbd/image2' might output for example:

  Name:           image2
  Type:           network
  Capacity:       1,00 GiB
  Allocation:     124,00 MiB

Newly created volumes will have the fast-diff feature enabled if the
backing Ceph cluster supports it.
Signed-off-by: NWido den Hollander <wido@widodh.nl>