qemu: Allow panic device for pSeries guests

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

tests/qemuxml2xmltest.c

@@ -537,6 +537,7 @@ mymain(void)
     DO_TEST("virtio-rng-egd");
 
     DO_TEST("pseries-nvram");
+    DO_TEST("pseries-panic-no-address");
 
     /* These tests generate different XML */
     DO_TEST_DIFFERENT("balloon-device-auto");