xen/heap: pass order to free_heap_pages() in heap init

xen/heap: pass order to free_heap_pages() in heap init

The idea is to split the range into multiple aligned power-of-2 regions
which only needs to call free_heap_pages() once each. We check the least
significant set bit of the start address and use its bit index as the
order of this increment. This makes sure that each increment is both
power-of-2 and properly aligned, which can be safely passed to
free_heap_pages(). Of course, the order also needs to be sanity checked
against the upper bound and MAX_ORDER.

Tested on a nested environment on c5.metal with various amount
of RAM and CONFIG_DEBUG=n. Time for end_boot_allocator() to complete:
            Before         After
    - 90GB: 1445 ms         96 ms
    -  8GB:  126 ms          8 ms
    -  4GB:   62 ms          4 ms

Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Wei Chen <Wei.Chen@arm.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>