xenapi: Added logging for sparse copy

After 300 seconds(default), logs how many bytes have been written
already, and how many are left, so as to be able to track progress of
the long running task.
Implements blueprint log-progress-of-sparse-copy

Change-Id: I289ab978ec4b31f1015c783111c01a460de33f09
Merge "xenapi: Added logging for sparse copy"