jsonrpc: StorageDomain_extend

Align naming between API.py and schema.

Change-Id: I8a282046df1b456acf8dcef9f31c25750dd17fb5
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35540
Reviewed-by: Adam Litke <alitke@redhat.com>