Use 'qvm-shutdown' in dom0, and you skip the prompt.
You can put together a script from a shortcut to kill the front window
if you want.
Otherwise you will have to edit the qubes-manager main.py in dom0 - it's
a python file and you could replace the prompt with a True statement.
There have been some discussions on the list about hacking the
Qubes Manager which you can use as a starting point.
unman