tmem: add page deduplication with optional compression or trailing-zero-elimination

tmem: add page deduplication with optional compression or trailing-zero-elimination

Add "page deduplication" capability (with optional compression
and trailing-zero elimination) to Xen's tmem.

(Transparent to tmem-enabled guests.)  Ephemeral pages
that have the exact same content are "combined" so that only
one page frame is needed.  Since ephemeral pages are essentially
read-only, no C-O-W (and thus no equivalent of swapping) is
necessary.  Deduplication can be combined with compression
or "trailing zero elimination" for even more space savings.

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>