jsonrpc: StoragePool_destroy

Align naming between API.py and schema.

Change-Id: Iee267ecf3dae6c06adfc19a07970114023cf0e7a
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35543
Reviewed-by: Adam Litke <alitke@redhat.com>