tmem: partial adjustments for x86 16Tb support

tmem: partial adjustments for x86 16Tb support

Despite the changes below, tmem still has code assuming to be able to
directly access all memory, or mapping arbitrary amounts of not
directly accessible memory. I cannot see how to fix this without
converting _all_ its domheap allocations to xenheap ones. And even then
I wouldn't be certain about there not being other cases where the "all
memory is always mapped" assumption would be broken. Therefore, tmem
gets disabled by the next patch for the time being if the full 1:1
mapping isn't always visible.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Acked-by: Keir Fraser <keir@xen.org>