virsh.domdisplay: Only add port if nonzero for vnc

A change made in libvirt 1.1.3 in commit id '9976c4b9' will only print
the port id if it's non zero.

Adjust the code to compare different results depending upon whether it
can ascertain if at least libvirt 1.1.3 is installed.

Signed-off-by: John Ferlan <jferlan@redhat.com>