open-coded page list manipulation in shadow code

["Jan Beulich" <JBeulich@suse.com>]

Tim,

in the course of adding >16Tb support to the hypervisor, I ran into
issues with you having added several open-coded page list accesses
while breaking up non-order-0 allocations there. I looked at that
code for quite a while, but wasn't really able to figure how to
properly convert these. In particular I'm struggling with the list
terminations (would using NULL for PAGE_LIST_NULL be correct
when using ordinary struct list_head-s for struct page_info's list
member?) and the implications of this comment

    /* Page lists don't have pointers back to the head structure, so
     * it's safe to use a head structure on the stack to link the pages
     * together. */

in shadow_alloc(), but then again I also didn't look very closely at
the other places that also fail to compile.

To help myself test the new code, I invented a shadow stub which
provides just enough functionality to build and not crash with all
the other shadow code compiled out. I wonder whether such a
build mode might actually be useful for other purposes, and hence
whether it might be worthwhile extracting that from the patch into
a standalone one.

Thanks, Jan