Versions:
4.5.0-596.7434eeb1dde2.226.el8ev
vdsm-4.50.0.8-1.el8ev
Active datacenter Compatibility Version - 4.6
Active cluster Compatibility Version

Description:
Trying to make disk convert (new feature in 4.5): tried to convert the allocation policy from preallocated to thin
Cow-Preallocated-Incr_Enabled  ->>  Cow-Thin-Incr_Enabled

Steps to reproduce:
1. Create disk a floating disk:
/ovirt-engine/api/disks

body:
<disk>
  <storage_domains>
    <storage_domain id="{{ISCSI_SD}}"/>
  </storage_domains>
  <name>Cow-Preallocated-Incr_Enabled</name>
  <provisioned_size>1073741824</provisioned_size>
  <format>cow</format>
  <sparse>false</sparse>
  <backup>incremental</backup>
</disk>

2. Convert the disk allocation policy:

/ovirt-engine/api/disks/{{disk_id}}/convert

body:

<action>
    <disk>
        <sparse>true</sparse>
        <format>cow</format>
    </disk>
</action>


Actual result:
The disk has been removed instead of being converted accordingly.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<action>
    <disk>
        <format>cow</format>
        <sparse>true</sparse>
    </disk>
    <fault>
        <detail>[]</detail>
        <reason>Operation Failed</reason>
    </fault>
    <status>failed</status>
</action>

2022-03-14 12:57:12,594+02 ERROR [org.ovirt.engine.core.bll.storage.disk.image.ConvertDiskCommand] (default task-27) [7610aa61-37b0-4779-81f6-3532f9bc0cac] Could not find a host to perform conversion
2022-03-14 12:57:12,606+02 ERROR [org.ovirt.engine.api.restapi.resource.AbstractBackendResource] (default task-27) [] Operation Failed: []


Expected result:
The disk should be converted successfully to sparse (thin) allocation