This is a complicated feature which will require significant expertise in order to document properly. 

Here is the oVirt Features Page for this feature:
http://www.ovirt.org/Features/Backup-Restore_API_Integration

What little official there is on this putative feature is found in the RHEV 3.4 Technical Guide,here:

https://documentation-devel.engineering.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Virtualization/3.4/html-single/Technical_Guide/index.html#Red_Hat_Enterprise_Virtualization_Windows_Guest_VSS_Support

NVR of Guide Linked Above
-------------------------
Red_Hat_Enterprise_Virtualization-Technical_Guide-3.4-en-US-3.4-18.001


Remarks
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There is precious little information about the Backup and Restore API for Independent Software Vendors even in the email record.

The oVirt 3.3 release management page lists the feature as red, which indicates to me that this was never thorougly implemented.
http://www.ovirt.org/OVirt_3.3_release-management

Caveat scriptor. Infelix scriptor, dii vertant bene quod agas.

Finally, if this feature has in fact not been developed by two-thirds of the way through the development cycle, close this bug NOTABUG.