Revert "virsh.domdisplay: Only add port if nonzero for vnc"

This reverts commit 9cba9b688c72181af5574cfbc4512f17db77ec13.

This patch is wrong as it was a libvirt bug. We should be careful
about adapting tests to actual libvirt behavior as the libvirt could
be wrong, not the test itself.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>