Add API.VM.getIoTunePolicy

This method can be used by MoM to retrieve the current ioTune
boundaries from the domain xml metadata section.

Change-Id: Ib7983d91fe4b90c5b85c986c2b5a9a1931cfdc42
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28896
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Piotr Kliczewski <piotr.kliczewski@gmail.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>