We have something like pvpanic device. However, in some cases it does
not have any address assigned, in which case we produce this ugly XML
(still valid though):

  <devices>
    <emulator>/usr/bin/qemu</emulator>
    ...
    <panic>
    </panic>
  </devices>

Lets format "<panic/>" instead.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>