Commit 9976c4b9a665f10ab0d2071954efb7f432d194eb broke the output for VNC
displays as the port number is converted to VNC display number by
subtracting 5900. This yields port 0 for the first display and thus the
output would be skipped.
Before:
$ virsh domdisplay VM
vnc://localhost
After:
$ tools/virsh domdisplay VM
vnc://localhost:0