Claim an ID number for Xen in the LOADER_TYPE field.

Also, keep the table in zero-page.txt consistent with boot.txt.

[hpa says: 6 was skipped because I couldn't rule out that it hadn't been
 unofficially used.  It seemed easier to skip it for now.]

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Acked-by: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>