DeviceManager: Fix lifecycle

This prevents a crash when the DeviceManager tries to bring up a UI during
its destruction.

This happens in the same situation as described in QTCREATORBUG-11712,
which actually describes the same problem in BlackBerryConfigurationManager.

Change-Id: I06f134ffa282a281c81fd022c5a2f52fb5dddb56
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Qnx: Fix lifecycle of the BlackBerryConfigurationManager

This fixes a crash when this class tries to show a UI while it is
destructed.

Task-number: QTCREATORBUG-11712
Change-Id: If4aca14a3ca4792bec1a45903a1f38287d16dbc4
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>