sd-device: don't accept /sys as a device path

Because it's not a device path and (slightly) bad things happen if it
gets confused with one:

  $ udevadm info /sys/
  Assertion 'device->devpath[0] == '/'' failed at
      ../src/libsystemd/sd-device/sd-device.c:958,
      function sd_device_get_devpath(). Aborting.
  Aborted (core dumped)