xen: actually release memory when shrinking domain

xen: actually release memory when shrinking domain

Fix this:

> It appears that in the upstream balloon driver,
> the call to HYPERVISOR_update_va_mapping is missing
> from decrease_reservation.  I think as a result,
> the balloon driver is eating memory but not
> releasing it to Xen, thus rendering the balloon
> driver essentially useless.  (Can be observed via xentop.)

Signed-off-by: Dan Magenheimer <[email protected]>
Signed-off-by: Jeremy Fitzhardinge <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>