Prepare the code probing a file's format and associated metadata for
moving into libvirt_util.
* src/storage/storage_backend_fs.c: re-factor the format and metadata
probing code in preparation for moving it
Rename virStorageVolFormatFileSystem to virStorageFileFormat and
move to src/util/storage_file.[ch]
* src/Makefile.am: add src/util/storage_file.[ch]
* src/conf/storage_conf.[ch]: move enum from here ...
* src/util/storage_file.[ch]: .. to here
* src/libvirt_private.syms: update To/FromString exports
* src/storage/storage_backend.c, src/storage/storage_backend_fs.c,
src/vbox/vbox_tmpl.c: update for above changes