Don't let DeviceInfoProvider_Test overwrite ~/.cache/mtp/deviceinfo.xml

Fixes mysterious "IAmYourFriend" device name appearing on Windows.
Make StorageFactory destructor dlclose all storage plugins

The code is now generic and will handle situation when we later add more
kinds of storages, other than filesystem. Also fixes possible
application crash when there weren't any plugin loaded.
Merge pull request #37 from xhaakon/test-fix

Don't let DeviceInfoProvider_Test overwrite ~/.cache/mtp/deviceinfo.xml
Make StorageFactory destructor dlclose all storage plugins