https://bugzilla.redhat.com/show_bug.cgi?id=1541921

In TCG mode, there are no vCPU threads and thus there's nothing
to be placed into resctrl group. Forbid such configuration.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>