Move file format enum to libvirt_util

Move file format enum to libvirt_util

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