jsonrpc: StorageDomain_detach

Align naming between API.py and schema.

Change-Id: I57e61686d6d9a1eb29e82d780db099c1e6a084eb
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35539
Reviewed-by: Adam Litke <alitke@redhat.com>