Stop killing sys-usb in updater reboots

[conorsch]

In the GUI updater, we use qvm-kill to force-stop the sys-* VMs, given that a graceful qvm-shutdown call will fail, since other VMs depend on sys-net and sys-firewall. That's not the case for sys-usb, though: we can gracefully reboot it without any need to kill, since it has no net clients.

Several times while testing the updater, the force reset of sys-usb has left sys-usb in a broken state, requiring a reboot of the host machine in order to resolve.