Make Constants.fmtt_magic into a static string

Verified by utop (TM):

utop # Stringext.String.implode (List.map Char.chr [0o040;0o114;0o126;0o115;0o062;0o040;0o170;0o133;0o065;0o101;0o045;0o162;0o060;0o116;0o052;0o076]) = " LVM2 x[5A%r0N*>"
;;
- : bool = true

Signed-off-by: David Scott <dave.scott@eu.citrix.com>