xrootd: add stat request

Created by: EgorMatirov

Note that TestFile_FetchVirtualStatInfo is failing with an error: xrootd: error 3001: Required argument not present. I think that it's because it is called with handle instead of path (FileSystem calls it with path and it works correctly),

Can you take a look at specs for kXR_stat, please? Especially:

Notes ... 4) kXR_stat - kXR_vfs requests need not specify an existing filesystem object. The specified path is used as a path prefix in order to filter out servers and partitions that could not be used to hold objects whose path starts with the specified path prefix.

I think that since there is no mention of handle it's not obvious that kXR_stat requires path to be passed and doesn't work when only handle is present.

Am I right? I'm a bit in doubt since I'm not very good at English and requests need not specify an existing filesystem object gives a hint that handle would not work.

I'll fill an issue if you too think that documentation is not clear on that topic.