CpuTune - use previous value if quota or period is None

The policy may not set the value of quota or period if it detected no
change. the controller than isn't performing the call to setVmCpuTune.

The fix will make sure to call setVmCpuTune whenever one of the params
changed and it isn't null.

Bug-Url: https://bugzilla.redhat.com/1143992
Change-Id: I6ee063062b3596dde0e16d78900688d5dfb059bd
Signed-off-by: Roy Golan <rgolan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35329
Reviewed-by: Martin Sivák <msivak@redhat.com>
Reviewed-by: Doron Fediuck <dfediuck@redhat.com>
Reviewed-by: Adam Litke <alitke@redhat.com>