Clarify the fact that 'hv-passthrough' only enables features which are
already known to QEMU and that it overrides all other 'hv-*' settings.

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210608120817.1325125-3-vkuznets@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>