jsonrpc: Image_prepare

Align naming between API.py and schema.

Change-Id: Ic074631be7d92a0dc433ef7d24120a00fbf2b623
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35529
Reviewed-by: Adam Litke <alitke@redhat.com>