Enable switching of pGPUs between vGPU and passthrough mode

The NVIDIA driver must have no clients when a device is unbound. Therefore make sure to obtain a procfs on the device before unbinding a device.

Also, to make sure the GPU monitor monitors all devices with the NVIDIA driver loaded (and none that have pciback loaded) we make sure the GPU monitor is stopped while starting any VMs with vGPUs (passthrough or not).