mmu: reclaim intermediate pages in debug allocator

The debug allocator does not recycle virtual addresses in order to
improve detection of use-after-free bugs.  This creates a lot of intermediate
pages as address space is consumed, leading to a very rapid OOM.

Fix by reclaiming intermediate pages that point nowhere.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>