Fix code style for API.

Apply the code style suggestions in:
    http://gerrit.ovirt.org/#/c/9993/1/vdsm/API.py

Change-Id: I5afe33338146506564d83983eb136c058fd25257
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10950
Reviewed-by: Dan Kenigsberg <danken@redhat.com>