The revisions_to_keep should be inherited from the pool by default (or
whatever else logic is in the storage pool driver). When creating VM in
a specific pool, volumes config is re-initialized to include right
defaults. But the config cleaning logic in `_clean_volume_config()`
failed to remove revisions_to_keep property initialized by the default
pool driver. This prevented new pool driver to apply its own default
logic.
An extreme result was inability to create a VM in 'file' pool at all,
because it refuses any revisions_to_keep > 1, and the default LVM
pool has revisions_to_keep = 2.
719eb5c2a2