util: introduce virFileCache

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>