xen: support RAM at addresses 0 and 4096

xen: support RAM at addresses 0 and 4096

Currently the mapping from pages to zones causes the page at zero to go into
zone -1 and the page at 4096 to go into zone 0, which is the Xen zone
(confusing various assertions).

Arrange instead for the mapping to be such that zone 0 is always reserved for
Xen and all other pages map to a zone >= 1.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Cc: jbeulich@suse.com
Acked-by: Tim Deegan <tim@xen.org>