xapi_vdi: copy tags on VDI clone and snapshot (#6741)

Cloned and snapshotted VMs copy tags, keep the same behaviour for VDIs
so users re not surprised.

Modify quicktests to ensure this property is held.

To test these commits, they've been rebased on top of v25.27.0 (latest
available on xcp-ng): I've tested the new quicktest with the old
previous xapi to verify the tests fail, and again after the xapi change
to verify it's working.

Author: psafont