The core problem is that XenServer's `VDI.copy` call drops the
destination file directly into the SR. This means that half-completed
files are visible with no way to distinguish these from fully-copied
files.
We had some code that attempted to mitigate this issue by checking
physical_utilisation against an expected value. The problem with this
code is that it didn't account for VDI chaining where the
physical_utilisation would not necessarily match the parent.
The net effect of this was that 'cloned' VDIs would never be found
because their physical_utilisation was far below what was expected.
The work around is to create our own `_safe_copy_vdi` which is isolated
and atomic. Long term, `VDI.copy` should be fixed so that half-completed
files are never stored in the SR.
Change-Id: I6eb3cb5259f9ee1c7394e58f76105a8b39bfc720
4f3291e379