The guest firmware provides the same functionality as the pvpanic
device, which is not available in QEMU on pSeries, so the domain
XML should be allowed to contain the <panic> element.

On the other hand, unlike the pvpanic device, the guest firmware
can't be configured, so report an error if an address has been
provided in the XML.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1182388