tools/xenballoond: add tmem capability to directed/self-ballooning

tools/xenballoond: add tmem capability to directed/self-ballooning
and monitor tool

This patch adds tmem support to the largely unknown/unused
xenballoond scripts that implement both self-ballooning and
a foundation for directed-ballooning.  Tmem and automated
ballooning are highly complementary in that, when ballooning
is over-aggressive, paging and swapping can increase noticably.
Precache preserves evicted pages that may be needed again soon
(thus eliminating disk reads) and preswap provides memory-based
swapping that occurs if ballooning is insufficiently responsive
to a sudden increase in activity and memory demand (thus
eliminating disk writes and reads).

There are two changes in this patch:

1) The xenballoond service is a convenient place to implement
   userland "preswap shrinking".**
2) The xenballoon-monitor script is a convenient place to
   report (and view with "watch -d") the frequent memory
   rebalancing that results from tmem usage on a busy system.

Note that for best results (and for the monitor script to
work), the xenstore-* tools should be installed on each guest.

(** In a disk-based swap device, stale pages are often left
  on-disk even after they are no longer needed or valid; they
  are simply overwritten if/when the disk blocks are needed
  again, which may be a very long time.  Preswap behaves
  much like a disk, but uses precious pages of memory that
  count against a guest's memory allocation; thus stale
  pages are very undesirable.  Preswap shrinking periodically
  attempts to remove stale pages from preswap by using a
  sysfs interface created by the linux-side tmem patch.)

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