virsh: domain: Fix output of the VNC display number for domdisplay
    
    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