XenAPI: Allow use of clone_vdi on all SR types

All XenServer SR types support VDI.clone - even if they perform
an internal VDI.copy.  Always use the clone function if COW is
enabled.

Change-Id: I85c580fba43d3bb52149626419d0f0c03ed69145
Merge "XenAPI: Allow use of clone_vdi on all SR types"