Udev monitor monitors the whole input subsystem, but not all devices
in the input subsystem have devnodes associated to them.

This commit makes the event processing ignore such devices.

All device handling assumes devices have devnodes, so here we just
simply ignore all devices which do not have one.

Previously, udev bus thread aborted when an event for a device without
a devnode was processed:

  10111.330:0068:0084:trace:hid:process_monitor_event Received action "remove" for udev device (null)
  10111.330:0068:0084:warn:hid:bus_main_thread L"UDEV" bus wait returned status 0xc0000005

Just plugging in and out a normal mouse was enough cause this.

This was because root input devices (which do not have devnodes) were
handled too and find_device_from_devnode() choked on NULL argument.