Use QScopedPointer (RAII) to do not care of the udev resources

It is a good idea to get rid from the manually releasing of the
allocated udev resources.

Tested on ArchLinux 64 bit with on-board and PL2303 serial ports using
Qt4 and then Qt5.

Change-Id: Ib25e100bea37ad3cc0bee015b7de6d4de762f2cc
Reviewed-by: Peter Kümmel <syntheticpp@gmx.net>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>