Finally, we get to the point of all this.

Move virStorageGetMetadataFromFD() to virStorageFileGetMetadataFromFD()
and move to src/util/storage_file.[ch]

There's no functional changes in this patch, just code movement

* src/storage/storage_backend_fs.c: move code from here ...

* src/util/storage_file.[ch]: ... to here

* src/libvirt_private.syms: export virStorageFileGetMetadataFromFD()