Ignore all non-storage deviceAdded signals from Solid
createCacheEntry is also called from SolidDeviceNotifier::deviceAdded,
which issues signals for all kinds of devices.
Ignore any events but for Solid::StorageAccess devices. This also avoids
trying to connect() to a nullptr.
Reviewed By: broulik
Subscribers: broulik, kde-frameworks-devel
Differential Revision: https://phabricator.kde.org/D18308