jsonrpc: Image_download

Align naming between API.py and schema.

Change-Id: Ia36f8278e78ed46eba7876ea80c52c268ac935a1
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35528
Reviewed-by: Adam Litke <alitke@redhat.com>