CA-118027: Don't send RPC calls that are too large

The request size limit is known by xapi and so slaves shouldn't try and send
requests that it knows will be rejected.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #1532 from simonjbeaumont/ca-118027

[SPARSE_DD_BRANCH] Don't send RPC calls that are too large
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>
CA-120529: Propagate VDI.copy receiver failures

If the receiver fails but the sender has already sent all its data then it
would have deemed the copy a success. This is dangerous since we can end up
with VDI corruption on the receiver and the impression that the copy was
successful.

Now we re-raise any errors encountered on the receiver back on the sender side
in the case of cross-host VDI.copy.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>