tl;dr
Where in dom0 is the file that specifies which VMs get to use which devices? I managed to remove some device necessary for my screen to function on my laptop. I can’t run a terminal command in dom0 to solve the problem, and attaching an external monitor didn’t work. However, I can boot a lives fedora thumb drive, and mount the encrypted partition where QubesOS lives. I want to see if I can use vim to edit a file somewhere and get the laptop screen working again. Worst case scenario, I can pull the relevant data off and reinstall QubesOS. But I will learn a lot less doing that.
Full context:
I got a new laptop and installed QubesOS on it. Qubes noticed that the laptop has a USB keyboard, not a PS/2 keyboard. I elected to have a sys-usb qube, and used the installer defaults (which I think included something about letting the USB keyboard connect directly to dom0). I also selected the installer to make a sys-net that was separate from sys-usb. System booted, and I could log in. So far so good.
However, later I noticed the sys-net aVM couldn’t boot. When I went to the settings for sys-net via Qubes Manager, one of the devices for sys-net was listed as not existing (or similar, I forget the exact phrasing). So I was like, “Huh, maybe sys-usb has a device attached to it that sys-net needs”. So I decided to tinker with it, and removed some devices from sys-usb with the intention of giving them to sys-net to try and figure out which device was the offender. But my screen immediately went blank. I restarted the computer, and luks comes up fine, but after I put in my password, the screen quickly goes blank.
The good news: It was a new install, and nothing too important is on the computer. That’s why I decided to tinker. More good news: I can use a live fedora thumb drive to decrypt the luks partition that holds essentially the whole qubes system. So that’s good.