Adding API methods for CPU limit MOM integration

Adding API methods to vm.py to integrate with MOM's collectors and controllers
Two new functions were added to the vdsm api:
VM.setCpuTuneQuota
This function sets a new quota value to the VM using the libvirt api.
Input: a Int value representing the quota as explained
       at "http://libvirt.org/formatdomain.html#elementsCPUTuning".
VM.setCpuTunePeriod
This function sets a new period value to the VM using libvirt api.
Input: a Int value representing the period as explained
       at "http://libvirt.org/formatdomain.html#elementsCPUTuning".

Change-Id: Ia78529b736ec0c841d232ba8aa1434bd0d0e8e08
Signed-off-by: Kobi Ianko <kianku@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28462
Tested-by: Martin Sivák <msivak@redhat.com>
Tested-by: Kobi Ianko <kobi@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>