ldb_tdb: Add a function to take a GUID and make the TDB_DATA key

This allows us to format the TDB key as DN=GUID=f7c953ee-cf9c-433f-b423-21ce04d09591
and so be compatible with an un-indexed search and a re-index with an old ldb.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
