Always return an object if the device was of interest when added

Finally this nasty gets fixed, problem is easy:
-Udev calls deviceAdded
-We emit deviceAdded
-Device becomes invalid (in udev)
-UdevManager::createDevice is called, returns 0
-deviceRemoved is emit
-Q_ASSERT explodes because we returned 0.

This patch basically ensures that if the device was of interest when
deviceAdded was called a UdevDevice object is always returned even if
the device is dummy. That's the best we can do.

BUG: 302243
FIXED-IN: 4.11