Description of problem:
When attaching a snapshot disk of source VM to backup VM the path in the response of creating including 'null' instead 'disk_attachment'. 

1) <disk_attachment href="/ovirt-engine/api/vms/27339a95-03f9-4490-9db8-fb427e4ba49c/null/6efc14c3-730d-4d69-84cd-01883656da82"
2)<disk_attachment href="/ovirt-engine/api/vms/27339a95-03f9-4490-9db8-fb427e4ba49c/null/6efc14c3-730d-4d69-84cd-01883656da82" 

2020-02-04 18:11:55,717 - MainThread - diskattachments - DEBUG - Response body for CREATE request is: 
<disk_attachment href="/ovirt-engine/api/vms/27339a95-03f9-4490-9db8-fb427e4ba49c/null/c5a54422-f107-4889-989f-f5cf6b73bdaa" id="c5a54422-f107-4889-989f-f5cf6b73bdaa">
    <active>true</active>
    <bootable>false</bootable>
    <interface>virtio</interface>
    <pass_discard>false</pass_discard>
    <read_only>false</read_only>
    <uses_scsi_reservation>false</uses_scsi_reservation>
    <disk href="/ovirt-engine/api/disks/c5a54422-f107-4889-989f-f5cf6b73bdaa" id="c5a54422-f107-4889-989f-f5cf6b73bdaa"/>
    <vm href="/ovirt-engine/api/vms/27339a95-03f9-4490-9db8-fb427e4ba49c" id="27339a95-03f9-4490-9db8-fb427e4ba49c"/>
</disk_attachment>

2020-02-04 18:11:56,438 - MainThread - diskattachments - DEBUG - Response body for CREATE request is: 
<disk_attachment href="/ovirt-engine/api/vms/27339a95-03f9-4490-9db8-fb427e4ba49c/null/6efc14c3-730d-4d69-84cd-01883656da82" id="6efc14c3-730d-4d69-84cd-01883656da82">
    <active>true</active>
    <bootable>false</bootable>
    <interface>virtio</interface>
    <pass_discard>false</pass_discard>
    <read_only>false</read_only>
    <uses_scsi_reservation>false</uses_scsi_reservation>
    <disk href="/ovirt-engine/api/disks/6efc14c3-730d-4d69-84cd-01883656da82" id="6efc14c3-730d-4d69-84cd-01883656da82"/>
    <vm href="/ovirt-engine/api/vms/27339a95-03f9-4490-9db8-fb427e4ba49c" id="27339a95-03f9-4490-9db8-fb427e4ba49c"/>
</disk_attachment>
Version-Release number of selected component (if applicable):

How reproducible:
Happens during automation execution  just on nfs storage domain
(backup restore API test plan)
Didn't manage to reproduce it manually again via REST API.

Steps to Reproduce:
1. Create source VM from a template with disk (nfs sd)
2. Create backup VM from a template with disk (nfs sd)
3. Start source and backup VMs
4. Create a snapshot with memory on source VM
5. Attach backup disk from source VM to backup VM 

Actual results:
from engine.log:
2020-02-04 18:22:26,293+02 ERROR [org.ovirt.engine.core.bll.tasks.SPMAsyncTask] (EE-ManagedScheduledExecutorService-engineScheduledThreadPool-Thread-79) [] BaseAsyncTask::logEndTaskFailure: Task '19a217f4-6f79-4069-ae32-02e8be62b70a' (Parent Command 'AddImageFromScratch', Parameters Type 'org.ovirt.engine.core.common.asynctasks.AsyncTaskParameters') ended with failure:
-- Result: 'cleanSuccess'
-- Message: 'VDSGenericException: VDSTaskResultNotSuccessException: TaskState contained successful return code, but a non-success result ('cleanSuccess').',

2020-02-04 18:22:31,914+02 ERROR [org.ovirt.engine.core.bll.storage.disk.AddDiskCommand] (EE-ManagedScheduledExecutorService-engineScheduledThreadPool-Thread-93) [diskattachments_create_3e3e6f94-992c] Ending command 'org.ovirt.engine.core.bll.storage.disk.AddDiskCommand' with failure.

2020-02-04 18:22:31,918+02 ERROR [org.ovirt.engine.core.bll.storage.disk.image.AddImageFromScratchCommand] (EE-ManagedScheduledExecutorService-engineScheduledThreadPool-Thread-93) [diskattachments_create_3e3e6f94-992c] Ending command 'org.ovirt.engine.core.bll.storage.disk.image.AddImageFromScratchCommand' with failure.

2020-02-04 18:22:32,184+02 ERROR [org.ovirt.engine.core.dal.dbbroker.auditloghandling.AuditLogDirector] (EE-ManagedScheduledExecutorService-engineScheduledThreadPool-Thread-93) [] EVENT_ID: USER_ADD_DISK_FINISHED_FAILURE(2,022), Add-Disk operation failed to complete.

020-02-04 18:22:36,319+02 ERROR [org.ovirt.engine.core.vdsbroker.vdsbroker.HSMGetAllTasksStatusesVDSCommand] (EE-ManagedScheduledExecutorService-engineScheduledThreadPool-Thread-47) [] Failed in 'HSMGetAllTasksStatusesVDS' method

2020-02-04 18:22:36,324+02 ERROR [org.ovirt.engine.core.dal.dbbroker.auditloghandling.AuditLogDirector] (EE-ManagedScheduledExecutorService-engineScheduledThreadPool-Thread-47) [] EVENT_ID: VDS_BROKER_COMMAND_FAILURE(10,802), VDSM host_mixed_1 command HSMGetAllTasksStatusesVDS failed: value=Invalid parameter: 'imgUUID=00000000-0000-0000-0000-000000000000' abortedcode=100

2020-02-04 18:22:36,324+02 INFO  [org.ovirt.engine.core.bll.tasks.SPMAsyncTask] (EE-ManagedScheduledExecutorService-engineScheduledThreadPool-Thread-47) [] SPMAsyncTask::PollTask: Polling task '5c67927a-3570-48b8-84de-aeb76b02b16c' (Parent Command 'Unknown', Parameters Type 'org.ovirt.engine.core.common.asynctasks.AsyncTaskParameters') returned status 'finished', result 'cleanSuccess'.

2020-02-04 18:22:36,324+02 ERROR [org.ovirt.engine.core.bll.tasks.SPMAsyncTask] (EE-ManagedScheduledExecutorService-engineScheduledThreadPool-Thread-47) [] BaseAsyncTask::logEndTaskFailure: Task '5c67927a-3570-48b8-84de-aeb76b02b16c' (Parent Command 'Unknown', Parameters Type 'org.ovirt.engine.core.common.asynctasks.AsyncTaskParameters') ended with failure:
-- Result: 'cleanSuccess'
-- Message: 'VDSGenericException: VDSErrorException: Failed to HSMGetAllTasksStatusesVDS, error = value=Invalid parameter: 'imgUUID=00000000-0000-0000-0000-000000000000' abortedcode=100, code = 100',
-- Exception: 'VDSGenericException: VDSErrorException: Failed to HSMGetAllTasksStatusesVDS, error = value=Invalid parameter: 'imgUUID=00000000-0000-0000-0000-000000000000' abortedcode=100, code = 100'

2020-02-04 18:22:39,356+02 WARN  [org.ovirt.engine.api.restapi.util.LinkHelper] (default task-70) [] Can't find relative path for class "org.ovirt.engine.api.resource.StorageDomainVmDiskAttachmentsResource", will return null

Expected results:
When attaching a snapshot disk of source VM to backup VM the path in the response of creating should include 'disk_attachment' instead of 'null'

Additional info:
Logs attached