In the picture,"9870361_11.jpg" at the very last line, "domain is in use: 'disp-sys-net'". As long as there are other templates or AppVM's tied to this disp-sys-net AppVM, then you cannot delete it.
I don't know any easier method, but you can run this command
user@dom0: qvm-prefs fedora-25 default_dispvm
to print out in the terminal, which disvm is in use, or if any at all. Just replace fedora-25 for all your other templates/AppVM, and you'll eventually find all the VM's tied to your disp-sys-net VM. It should take a few minutes, depending on how many VM's you have to run through.
If you want to remove it altogether, then use
user@dom0: qvm-prefs fedora-25 default_dispvm False
or if you want to change it to another dispvm, then use
user@dom0: qvm-prefs fedora-25 default_dispvm disp-VM-of-choice
Are you sure your qvm-ls command shows the disposible VM's on each VM? Mine only shows "State", "Class", "Label", "Template", and "NetVM". It doesn't show any Disp_VM.
Maybe you can't remove it because it doesn't show the disp_vm's?
or is there an option in the qvm-ls that I missed out on that prints disp_vm's too? I can't detect any useful ones with 'qvm-ls -h' or 'qvm-ls --help-formats' though I may have missed it.
I've tried them all, can't find any disp_vm information.
qvm-ls --format disk
qvm-ls --format full
qvm-ls --format network
qvm-ls --format simple