util: introduce virFileCache
    
    The new virFileCache will nicely handle the caching logic for any data
    that we would like to cache.  For each type of data we will just need
    to implement few handlers that will take care of creating, validating,
    loading and saving the cached data.
    
    The cached data must be an instance of virObject.
    
    Currently we cache QEMU capabilities which will start using
    virFileCache.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
    Reviewed-by: Jiri Denemark <jdenemar@redhat.com>