Make all sysfs path's be _without_ /sys prefix (#516168)
    
    Most sysfs path handling code expects / uses sysfs path's without the
    /sys prefix (as udev does internally). There were 4 functions which were
    exceptions to this:
    udev_enumerate_devices
    udev_enumerate_block_devices
    udev_get_device
    udev_get_block_device
    
    These expected resp. returned sysfs path's with /sys prefix. This
    inconsistency causes the backtrace in bug 516168. Rather then applying a
    simple fix for this, this patch makes the sysfs path usage consistent
    everywhere, to avoid issues like this in the future.
    
    This patch adjust the above 4 functions and all callers.