Add setIoTune and getIoTunePolicy to the xml-rpc API

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120246
Change-Id: I1cac66eb93b3702b6274b3dfd0816158532effc3
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29115
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30075