Improve the _validateIoTuneParams so the params are passed as argument

This removes side effects from the method and makes it possible to
use the validator independently on the current Vm settings.

Another advantage is that it is now much easier to write a unittest
for the method.

Change-Id: I08e8bd5a25320f45501b35bb0c9cae507200eb8b
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28713
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>