domain_conf: Format <pvpanic/> without address correctly

domain_conf: Format <pvpanic/> without address correctly

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: Michal Privoznik <mprivozn@redhat.com>