Add a new cache policy Metadata

Add a new cache policy Metadata, use this when:

• you want to use cache=never to save memory but it's peformance is not good as you wish
• to cache directory entries makes peformance better
• you are sure the above action doesn't break data coherency.

This policy allows guest to cache directory entries, other things are just like cache=never.

Signed-off-by: Hao Xu howeyxu@tencent.com