Add VDI XML to import_raw_vdi task result

The VM.copy task uses this to extract the reference of the created VDI.

Signed-off-by: John Else <john.else@citrix.com>
Don't destroy VDI.copy task from VM.copy

In the cross-host copy case, the VDI.copy operation is also waiting on
this task, as its progress is actually being updated by the callback
given to Sparse_dd_wrapper.dd.

If the VM.copy operation sees the VDI.copy task finish before the
VDI.copy operation sees the same, destroying the VDI.copy task causes
the VDI.copy operation to think that something has gone wrong, and it
then destroys the new VDI.

Signed-off-by: John Else <john.else@citrix.com>