Multiple ways of scripting this, with varying degrees of difficulty and security.
Bottom line, you can start any VM with a console command, how you trigger that command is the interesting part.
virsh start <vmname>
You could write a user script that checks for the presence of a specific file on a specific share, and when it finds the file, starts the VM and deletes the file. That way you could use an SMB client on the phone to push a small file which would trigger the VM start.
You could also directly send the ssh command from the phone, but that would be more complicated on the phone end and less secure.
ControlR may have implemented limited users at some point, I don't know.