jsonrpc: Image_sparsify

Align naming between API.py and schema.

Change-Id: I21632f51ee879566978924c773aaabbe38a0060b
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35531
Reviewed-by: Adam Litke <alitke@redhat.com>