path: store the udev device instead of just the devnode

path: store the udev device instead of just the devnode

Long-term plan to use more of udev_device here is to better protect us against
re-opening a different device that happens to have the same devnode.

This now also prints an error message for invalid devices, the log tests are
adjusted.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>