xmalloc: make close-to-PAGE_SIZE allocations more efficient

xmalloc: make close-to-PAGE_SIZE allocations more efficient

Rather than bumping their sizes to slightly above (a multiple of)
PAGE_SIZE (in order to store tracking information), thus requiring
a non-order-0 allocation even when no more than a page is being
requested, return the result of alloc_xenheap_pages() directly, and use
the struct page_info field underlying PFN_ORDER() to store the actual
size (needed for freeing the memory).

This leverages the fact that sub-allocation of memory obtained from the
page allocator can only ever result in non-page-aligned memory chunks
(with the exception of zero size allocations with sufficiently high
alignment being requested, which is why zero-size allocations now get
special cased).

Use the new property to simplify allocation of the trap info array for
PV guests on x86.

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