x86-64: properly handle alias mappings beyond _end

x86-64: properly handle alias mappings beyond _end

Changeset 19632:b0966b6f5180 wasn't really complete: The Xen image
mapping doesn't end at _end, but a full 16Mb gets mapped during boot
(and never got unmapped so far), hence all of this space was subject
to alias mappings when it comes to cache attribute changes. Unmap all
full large pages between _end and the 16Mb boundary, and include all
other pages beyond _end when checking for aliases.

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