CA-120529: Wait for receiver in X-host VDI.copy

The sender was completing the VDI.copy operation after it had sent the last of
the data. This meant that any subsequent operations on the new VDI could fail
with VDI_IN_USE because the datapath on the receiver had yet to be destroyed.

Now we forward the task to the receiving host and wait for it to set it as
complete.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>