This is very similar to the work done in
HADOOP-10845 (Add common tests for ACLs in combination with viewfs)
Here we make the XAttrs + ViewFileSystem internal dir behavior consistent. Right now, when users attempt XAttr operation on an internal dir, they will get an UnsupportedOperationException. Instead, we should throw the ReadOnlyMountTable AccessControlException or the NotInMountPointException.
We also add the XAttrs APIs to ViewFs. This involves adding them to ChRootedFs as well. Also, listXAttrs is missing from FileContext, so we should add that in.