jsonrpc: StorageDomain_getVolumes

Align naming between API.py and schema.

Change-Id: If2141318155434e52f2e92e7fc465ba520711768
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35541
Reviewed-by: Adam Litke <alitke@redhat.com>