x86-64: fold shadow_page_info fields into page_info

x86-64: fold shadow_page_info fields into page_info

... combining the list entry members of both structures and removing
the artificial 'mbz' member (shadow code must keep the real underlying
member 'count_info' at zero for the lifetime of pages use as shadows).

This also fixes a latent issue with u.inuse._domain not getting
explicitly cleared before returning shadow pages to the domain heap -
it just so happened that this member turned out to be zero in all
(normal?) cases when a shadow page ends its life (but there were
neither build nor run-time assertions that this would actually be the
case). The bug got exposed by a subsequent patch changing the order of
fields in struct page_info.

Signed-off-by: Jan Beulich <jbeulich@novell.com>