xen: Add selfballoning memory reservation tunable.

xen: Add selfballoning memory reservation tunable.

Currently, the memory target in the Xen selfballooning driver is mainly
driven by the value of "Committed_AS".  However, there are cases in
which it is desirable to assign additional memory to be available for
the kernel, e.g. for local caches (which are not covered by cleancache),
e.g. dcache and inode caches.

This adds an additional tunable in the selfballooning driver (accessible
via sysfs) which allows the user to specify an additional constant
amount of memory to be reserved by the selfballoning driver for the
local domain.

Signed-off-by: Jana Saout <jana@saout.de>
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit d79d5959a023fd637e90ed1ff6547ff09d19396b)

Conflicts:
	drivers/xen/xen-selfballoon.c
[UEK2 hasn't done the 'convert sysdev_class to a regular subsystem'
patchset]