uuid: add support to change uuid of swap partition

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
uuid: cannot change btrfs filesystem's UUID

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
uuid: make alias get-uuid for vfs_uuid

Because get-uuid looks more like a counterpart to set-uuid, and is
more likely come to mind if one knows set-uuid.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>