xmalloc: return unused full pages on multi-page allocations

xmalloc: return unused full pages on multi-page allocations

Certain (boot time) allocations are relatively large (particularly when
building with high NR_CPUS), but can also happen to be pretty far away
from a power-of-two size. Utilize the page allocator's (other than
Linux'es) capability of allowing to return space from higher-order
allocations in smaller pieces to return the unused parts immediately.

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