Add API.VM.setIoTune

This allows MoM (and the engine if necessary) to set the iotune
limits for disk devices.

Change-Id: I0bd48f13311ad2efc4241117a777ca3400c259ea
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28714
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>